From the popular perception of vanilla as the ordinary, bland flavour of ice-cream:
– the phrase plain vanilla has come to be used figuratively to mean having no special or additional features, ordinary, basic;
– the noun vanilla has come to be used attributively to mean plain, ordinary, conventional, safe, unadventurous.

 

1: EARLY OCCURRENCES OF PLAIN VANILLA

 

The earliest figurative use of plain vanilla that the Oxford English Dictionary (third edition, 2006) has recorded dates from 1959.
I have, however, found earlier occurrences.

 

1.1: 1931 TO 1933—PLAIN VANILLA: OBSCURE OR UNCERTAIN MEANINGS

 

Three of the earliest figurative uses of plain vanilla that I have found occur in the horse-racing column Collyer’s Comment on the Sport of Kings, by Bert E. Collyer. However, the meaning of the phrase is unclear.
—This is what Bert E. Collyer wrote in his column:

1-: published in The Courier-Journal (Louisville, Kentucky) of Wednesday 23^rd September 1931:

From Fairmount comes the word that Linmast, in the second, and Peter in the third, will turn the trick for followers of the “daily double.” As a matter of fact, the lad there makes Peter his best bet and adds, “Tell ’em all to clamber on this one; will run right back to that good effort of last time.” Make mine plain vanilla.

2-: published in The Cincinnati Enquirer (Cincinnati, Ohio) of Friday 29^th January 1932:

Fair Grounds jockey mounts:
Paccuma—Captain Ferry, Glidelia.
Garner—Beauty Bride, Sistery and Herendeen.
[…]
Elston—Kay Frances, Chene, Smear, Jane Packard, Master Ogden and Shakelford.
Just plain vanilla.

3-: published in The Cincinnati Enquirer (Cincinnati, Ohio) of Wednesday 8^th June 1932:

Tennywood, in the second, is another consensus horse that seems ready for the question. As for Monde in the fourth, I fancy you may get as much as 5 to 1 and your own dough back. Catch on?
I’ll take plain vanilla. More anon.

Another early figurative use of plain vanilla occurs in the column Sport Situation, by Homer Olsen, Austin Statesman Sport Editor, published in The Austin Statesman (Austin, Texas) of Friday 13^th November 1931—here, too, the meaning of the phrase is unclear:

In order to go up against the Southern Methodist Mustangs come Nov. 28 with everything square, the Frogs must win that game here Saturday. If they lose to the Longhorns, then it won’t make a great deal of difference if they go ahead and beat the Ponies. For then the mess will be tied, and that’s plain vanilla for everybody. Surely nobody wants it to come out that way.

The phrase seems to mean straightforward in an article about a forthcoming wrestling contest, published in the Tyler Morning Telegraph (Tyler, Texas) of Tuesday 29^th March 1932:

The next match from point of attraction on the triple header program is Joe Parelli, the barefooted Italian, versus Walter Stratton, the Chicago strong boy. Both jousters have a strong leaning toward the rough and dirty things of life, in fact they practically confine themselves to tactics that run the referee nuts and keep the crowd yelling. Stratton last appeared here with Silent Condell and Parelli has had trouble here lots of times. Rules of this contest will be just plain vanilla.

The meaning of the phrase is obscure in the following from The Burlington Free Press (Burlington, Vermont) of Wednesday 21^st September 1932:

Bonus Resolutions—in the Japanese Manner

A group of veterans representing a fraction of the war veterans in this city met in closed session Monday evening and brought forth a bonus resolution worded like an Oriental document. Such minority groups do well to state their reasons. Since they have designs on the public treasury, contrary to the early ideals of their organization, the public is entitled to know why.
If the resolutions of Burlington Post, just passed, are labelled patriotism, common sense, or fair play,—then we’ll take plain vanilla.

The phrase plain vanilla may mean having no special or additional features in this cartoon, from On the Spot: Being Odd Angles on the Week’s News, by Croslin (?), published in the Forth Worth Star-Telegram (Fort Worth, Texas) of Sunday 8^th January 1933:

 

1.2: 1934 TO 1937—PLAIN VANILLA: ORDINARY, BASIC

 

In her column On With The Show, published in The Pittsburgh Press (Pittsburgh, Pennsylvania) of Tuesday 15^th May 1934, Florence Fisher Parry used plain vanilla to characterise the U.S. singer and actor Bing Crosby (Harry Lillis Crosby – 1903-1977), who starred in We’re Not Dressing (1934), a musical comedy film based on The Admirable Crichton (1902), a stage play by the Scottish dramatist and novelist James Matthew Barrie (1860-1937):

If you can imagine Bing Crosby as an Admirable Crichton—well, anyway, YOU be the one to imagine it. Voice or no voice, he’s plain vanilla to me.

In her column from the Kelvin Kitchen, published in The Adair County Democrat (Stilwell, Oklahoma) of Thursday 2^nd August 1934, Joan Adams played on the figurative and literal meanings of plain vanilla:

GILDING THE ‘VANILLY’

“We’re dressing”, say all the lovely ice creams this midsummer. And those white as the driven snow are saying it with special emphasis, for they have been termed “just plain vanilla” long enough! It’s like a game, to turn a plain unadorned scoop of ice cream into something provocative to the eye and irresistible to a voyaging spoon. So let’s mantle our ice creams in chocolate fudge this summer, or mount it in marshmallow and sirupy fruits, or top it with gay cockades of raspberry sauce.
[…]
[…] With such artifices for gilding the lily, who shall say, “just plain vanilla”?

The following is from Thumbnail Reviews of New Movies, by Colvin McPherson, published in the St. Louis Post-Dispatch (St. Louis, Missouri) of Saturday 15^th December 1934:

“Have a Heart” is a plain vanilla love story about a little girl who isn’t lame, is and then isn’t.

The phrase plain vanilla clearly means ordinary in this article published in the Boston Evening Globe (Boston, Massachusetts) of Saturday 13^th July 1935:

HIS WIFE’S OLD FRIEND SIZED UP BY HUBBY

All through the first year of their married life a Brooklyn bride never mentioned to her husband that she ever had been seriously interested in any other man.
Then one night she announced that an old friend was coming to dinner. She assured her husband that the old friend was just that, and nothing more, but somehow he became silent.
As they waited for the guest she told what an exceptionally fine man the old friend was, how intelligent, how sensible, how considerate, how mysterious, how altogether far above the average he was.
The guest came, they dined, they talked and the guest departed.
It was some time before the husband of a year’s standing would answer his wife’s question as to how he liked the guest. Finally he worked his lips for several seconds and blurted out:
“Just plain vanilla.”—New York Sun.

Harold W. Bentley recorded a very specific sense of the phrase in Linguistic Concoctions of the Soda Jerker, published in American Speech (Columbia University Press) of February 1936:

PLAIN VANILLA. Soda man once manager and now behind the counter.

The phrase occurs in this advertisement, published in the Forth Worth Star-Telegram (Fort Worth, Texas) of Sunday 3^rd May 1936—the Blue Bonnet was one of the participating beauty salons:

Centennial Beauty Campaign
The Following Beauty Parlor Advertisers offer:
$20 CASH FIRST PRIZE
$10 CASH SECOND PRIZE
For Best Verses Submitted

Get entry blank for this contest with each 50c purchase from one of the Beauty Parlors listed below . . .
TO THOSE who are just plain vanilla,
But would like to be sasparilla,
I give my advice today—
“Go the Blue Bonnet Way!”
—Entry of La Delle Chapman.

Finally, the following is from the St. Louis Globe-Democrat (St. Louis, Missouri) of Friday 8^th January 1937:

Absent-minded husbands of the Optimist Club of St. Louis who have been in the habit of committing errors of omission and commission on the one day of the year that the missus bats 1000 in memory tests, her wedding anniversary, will hereafter be given an opportunity to mend their ways in a plan devised yesterday by the organization.
The scheme which is optimistically aimed to end all domestic troubles in the organization, calls for a reminder of wedding anniversaries sent three days in advance of the one day the little lady likes to think of in terms different than just plain vanilla.

 

2: EARLY OCCURRENCE OF VANILLA SEX

 

According to the Oxford English Dictionary (draft additions, 1997), the figurative uses of vanilla originally referred to sexual activity, especially in vanilla sex. The earliest occurrences that this dictionary has recorded are from The Queens’ Vernacular: A Gay Lexicon (San Francisco: Straight Arrow Books, 1972), by Bruce Rodgers:

Vanilla bar, a gay bar that is not SM.
Vanilla, […] rigid, conforming, goody-goody ‘This neighborhood is too vanilla for the licks of us.’

I have, however, found an earlier use of vanilla sex in The Latest Offerings in Books and Magazines, published in the Hilo Tribune-Herald (Hilo, Hawaii) of Wednesday 26^th October 1960:

Ever watch your teen-agers choosing something to read at a magazine rack?
[…]
We bought some books and magazines more or less at random the other day.
[…]
If you want to drop down into the thirty-five-cent range of books you may buy, as we did, copies of “The Hayloft” and “Song of the Whip.”
“The Song of the Whip” is the tale of “a lady bullfighter incapable of normal desire” and of “immorality in the guise of recreation,” according to a blurb on the back cover.
It contains philosophical writings. Take this, for instance, as spoken by one of the characters—and we do mean characters—in the book:
“. . . there are different flavors of sex just as there are of ice cream. Vanilla is the old stand-by, the most common, the average, the one everyone is sure to like. Thus most sex is ‘vanilla’ sex—good, sturdy, dependable. But the real connoisseur tries various flavors—chocolate, strawberry, and so on.”