Sheen's a great item for augmenting combo champions like Vayne, Poppy, Irelia... and On hit champions like Nasus and Jax...

Early game it helps. Don't rely on it much mid or late game

I find spending a bit more for brutalizer and then turning it into youmuu's ghostblade works much better than going sheen to triforce. Armor pen + cooldown reduction + damage > mana (which you don't really need on vayne) + ability power (don't need on vayne) + damage proc (useful, but doesn't make it worth the 1260 gold).

Ghostblade gives you so much more useful things that I can't see not getting it on vayne.

I wouldn't go with sheen unless I know that a Trinity Force is a absolute must to win the match I'm in. I find that for vayne in particular, you need to have some kind of back-up if you want to survive in team fights. Especially when your team can't help you in the middle of their team battle.