Jinx and vayne now both have positive winrates, despite being explicitly late game carries.

You're telling me, they need to lose more matches than they win matches and only then will they become balanced?

Perhaps you're failing to realize their (vayne specifically) winrate only goes higher than 50 after 30 minutes?

not allowed to win if you play jinx or vayne ?

That's because the meta shifted so that matches last longer, which makes late game champions more relevant. Early season was all about lanning phase and snowballing so matches ended or at least were decided pretty soon. At some point the average match was 22 mins long. Late game carries had no room in that meta.

Also Jinx's buff to her ult was pretty minor, and she had a nerf to her chompers to compensate.

Heres a post that Gypsylord made about why they balance Jinx with having an above 50% winrate

http://boards.na.leagueoflegends.com/en/c/gameplay-balance/BiL98QHi-this-is-a-disgusting-excuse-for-an-olaf-riot-jinx-pbe-changes?comment=00070005

As for Vayne, I actually do think she's too strong rn. She was designed to build attack speed to synergize with her silver bolts, but her most popular build is designed to deal a ridiculous amount of damage in only a few auto attacks.