Well its just my opinion. But as far as I've seen any adc who understand how / when to trade, will push vayne off farm easily making her gimp until the late game. Of course if you get her to the late game she will become a power house, but a lot of the time by that point its too late.
Though graves has low range just like vayne, he is bursty, and not sustained damage like vayne. So when a long ranged adc like cait or jinx pokes him, you can turn and burst them for an even or better trade and E out.
Also with graves, if you get even slightly ahead in build... such as a BF sword when they have a pickaxe... You can go HARD and mess up that bot lane, particularly with a tanky initiate like leona. Not to mention you bring huge aoe damage to your team which is really good with most comps.
However... with all this being said, it all depends on team comp. I pick my adc based off team comp, but seldom is that Vayne for me.
Graves is countered by Cait hard though if she knows how to use her range to poke then E away to avoid the graves retaliation. So I don't recommend going him against her. At lower elo its not a problem cause most adc's blow, but later I can see it becoming rough.
But enough jumping on the Gravy train.
I just think he works REALLY REALLY well in the current meta.
Adc is all about understanding how to trade and shut down the opposing adc's farm and he's been working really well for me.
Graves, Corki, and Sivir are the best atm in the current meta IMO.