Corki or vayne? (poll)

Both are viable. Corki is more 'in meta' right now. He is a brutal lane bully with an uber-powerful mid game spike when he finishes Trinity. He is also a poke monster.

Vayne is a late game hyper carry split pusher Marksman/Assassin hybrid thing. She didn't get buffed by the recent marksman changes, because she goes BotRK+Shiv(Or PD) early. Most people going for late game power right now will play Tristana.

Corki since you're more likely to dominate lane.

It depends on how good you are with Vayne. If you are OP with Vayne, then Vayne will ALWAYS be better as you can make insane outplays. As you're asking which is better, though, I'm feeling you aren't a godlike Vayne, so you should probably go with Corki as he is good at all points in the game. He only real weak point is his late game as he has inferior damage/range compared to Trist/Kog/Twitch. He also is useful if your top/mid want to be AD, as he is the only ADC that has a good amount of magic damage.