With reference to the use of Vaseline to ease anal intercourse, and based on the alliteration in V, the colloquial Australian-English phrase Vaseline Valley designates a stretch of Oxford Street, in Sydney, which is the city’s main gay district.
—Cf. also the following Australian-English phrases:
– to get off at Redfern, meaning to practise coitus interruptus, with reference to Redfern, a train station positioned one stop before Sydney Central Station;
– like a Bondi tram, meaning speedily, with reference to the tram service between Sydney and Bondi Beach, a popular beach located 4 miles east of Sydney city centre;
– Double Pay, i.e., Double Bay, an affluent harbourside suburb of Sydney.

These are the earliest occurrences of the phrase Vaseline Valley that I have found, in chronological order:

1-: From The gay world runs hot: Capriccios—an old flame, by Richard McGregor, published in The Sydney Morning Herald (Sydney, New South Wales) of Saturday 10^th April 1982—Tommy Brown was the manager and part owner of Capriccios, which had just been destroyed by fire:

Tommy Brown hopes people will remember Capriccios for the costumes and the shows.
The sequined costumes, that is, that adorned its famous drag shows in its salubrious premises upstairs in Oxford Street. The local drag show with an international reputation.
[…]
When Capriccios, or International Vanities as it was sometimes known, opened its doors in the early 1970s, it started the gay development along the stretch of Oxford Street that is now known as “The Golden Mile”, or, in a cruder vernacular form, as “Vaseline Alley”.

2-: From the interview of Sue Cornwall, who was then making a documentary film titled Kids In Trouble—interview by Gill Leahy, published in Filmnews (Sydney, New South Wales) of June 1985:

You said it was your first chance for many years to make a serious film. What do you think about documentary now—what’s in the future for you?
I would like to film a couple of kids that have emerged in this film again, one of them turns eighteen in July, he’s a heroin addict, and a prostitute, he’s been committed to a training institution, which is a euphemistic term for a kid’s gaol, but he’s been on the run. I think there’s a very good chance that he will end up in Long Bay ¹ when [he] turns eighteen in July. In this film we’ve got a lot of references to Long Bay, Vaseline Valley, the Big House ², he jokes about it quite often, how he’s going to end up there.

¹ This refers to Long Bay Correctional Centre, in Malabar, a suburb of Sydney.
² Big House is slang for prison.

3-: From the column On the inside, by Kevin Perkins, published in The Sun-Herald (Sydney, New South Wales) of Sunday 2^nd February 1986:

Cold comfort
Michael Yabsley ³, Liberal MP for Bligh in Sydney’s erogenous inner-city zone, was given the Captain Cook award by Nifty recently for discovering prostitutes in The Cross (and because Bligh takes in Oxford St, Mike Cleary ⁴  calls him the Member for Vaseline Valley). Anyway, Michael, whose home at the Loo is surrounded by brothels (bordellos, please!), says things are just as busy as ever on the beat, despite AIDS etc. But commercially inclined ladies haven’t disported under his balcony since he threatened to tip a bucket of iced water over them.

³ Michael Yabsley (born 1956) was then a Liberal member of the New South Wales Legislative Assembly.
⁴ Michael Cleary (born 1940) was then a Labor member of the New South Wales Legislative Assembly.

4-: From My suburb right or wrong, published in The Sydney Morning Herald (Sydney, New South Wales) of Thursday 6^th March 1986:

Kathy Lette ⁵
Surry Hills
Writer Kathy Lette describes the Surry Hills house where she and her husband Kim Williams live as “anorexic—very tall and very thin. It’s in the famed Vaseline Valley. It’s human minestrone—junkies, trendoids, prostitutes, computer people, bureaucrats—the lot.”
Kathy lived in Balmain before but tired of “skiing on dog excreta” and “people dropping their names from great heights”. Surry Hills is more incognito “but also because it’s the first time in my life that I’ve lived where there are no cockroaches. We’re very close to Paddo and the cockroaches choose to stay there. They prefer pate to Promite ⁶”.

⁵ Kathy Lette (born 1958) is an Australian-British author.
⁶ Promite is a savoury spread made from yeast extract and vegetable extract, similar to Marmite and Vegemite.

5-: From Girls’ Night Out (Sydney: Pan Books, 1987), by Kathy Lette:

Back in Oxford Street Mouche sat down on the gutter and doled out our busking money into nineteen silver piles, five coins high with thirty-two brown cents left over. As I waited for the final count, I cross-examined her about the call. Mouche was like a used tube of toothpaste. Squeeze as hard as you could, you still wouldn’t get a drop out of her.
‘We’re rich,’ she laughed up at me. Passers-by responded with sideways glances of intrigue at Mouche’s technicolour hair. Lately her head had looked like the cover of a Heavy Metal record album. She nuked every optic nerve in sight.
‘But why didn’t they confiscate the money?’ We were deep in the heart of Vaseline Valley, so I automatically pressed the ‘Walk’ button with my elbow.
‘Oohh . . . watch it,’ she mocked, ‘you might get AIDS of the funny bone.’
Embarrassed, I tried to turn my akimbo thrust into a nonchalant lean.