whos a better adc?

Right now? Vayne, assuming no prior skill with either character and equal build points.

I always liked Twitch over Vayne, but since Vayne got her silver bolts buffed I would have to say Vayne.

I suck with both.

That said I have also seen both of them destroy teams.

Each has their own flair in doing this.

I think twitch is better for back doors and taking objectives and figure vayne is over all more dominating in team fights.