You're a low dmg top laner, and the enemy sends Vayne up there, what do you do?
Apart from beg for a trade(then probably not get it, and have one of the most annoying possible laning phases hehehe). A half decent Vayne will never get ganked unless your jungler is really slick, she'll eternally kite the poor melee guy shes laning against, and she will be scaling faster offensively than you can keep up with defensively. Her kit pretty much guarantees that the top laner will never out trade her unless she does something stupid, so you just have to give her the lane, getting whatever measly cs you can off spamming skills under the turret. It doesn't take long for her to get out of control when she's practically free-farming top lane, and when it's a Vayne that's a serious problem.
I'm not fully convinced she's balanced anyway, I mean if someone can take an adc top lane and start winning trades with someone like Maokai in no time, I really feel like something is wrong here. Are ADC's really supposed to be that self-sufficient? She can peel for herself, chase endlessly, dashes every few seconds, and when all else fails she pops the ult and vanishes. Her kit is just too much for any melee champ that can't pull a Xin Zhao on her. The W gives her much higher damage output than her opponent(unless they're Riven or something), the passive makes sure if the enemy runs close enough to get shot once, they're getting shot all the way back to the turret, and her Q+E pretty much guarantees that you won't be catching her with anything unless she bricks something.
Anyway, balance aside, what's the best thing to do in this situation? Within a few minutes the Vayne will be making the top laner pay for any cs with huge chunks of health, and you can only safely farm under turret if she doesn't stick around to keep poking you(which she can do pretty safely with a ward up). Do I just go to a different lane anyway and try to help them? Or do I become the 2nd jungler? I know giving away top lane is absolutely terrible, but is it worse than getting almost no cs and risking feeding the Vayne even more?
and early defensive masteries go a long way. Also, she has 0 wave clear. You need to punish her with CS for hitting you (i.e. make yourself a target when a minion is about to die). Force her to choose between hitting you and getting a CS. Abuse brush, she has no way to check it herself. She used trinket? Good, no protection from ganks. Let the jungler know immediately.
You said melee, so
will work well in nullifying. Pretty much you want to just be tanky in a non-maximum health capacity.
Hope that helped some. And yes, a support item that regens so strongly is very valuable.
cannot be understated.