Some dance records are still released on vinyl, mostly for the nightclub dj market. So it could be a more recent No1 than you think. Also, some other records get a limited vinyl release.
It's no help to you, but I seem to remember one of Roxette's (***** Swedish band) singles was the first to go to number one without being issued on vinyl. It was only released on CD and Cassette.
"Call on Me" was certinaly available on 12". You'd be surprised what's out there, I found a Girls Aloud 7" picture disc in HMV the other day! Roxette never had a number 1 single in the UK, but it was some crud like that. Possibly Celine Dion's "Think Twice" in 1995, I remember there being a bit of a fuss about it.