Fervor of Battle is a nice Keystone when it comes to the Mid/End Game. While laning, it dosnt really help you much. If you want a stronger laningphase, you can take thunderlords and when you think that you will get bullied on lane you can take Warlords, but i dont recommend those two. Fervor is just too strong.

I recoment putting 3 points in q at the start, because you dont get much value from your w the first 10 minutes. That synergieses with the bf-sword start. When you have your bf, try to poke your opponent with one Auto Attack with your q and then go back, wait for your q and do it again.

I wouldnt say there are matchups which are undoable. There are matchups that are harder to survive. I would try to not fight till you have your bf and zeal/Phantom dancer. When your even with your lane oppent till then I think you can 1 v 1 Every Enemy ADC exept Quinn: (Her blind is just too strong). Against good matchups you can try to play aggresive if you feel confident (Against sivir, ereal,...)

Vayne is really hard to play in teamfights when you dont have a Team that can peel for you. When this is the case, you should play very defensive and wait till your team engages/gets engaged. You should wait till everyone is fighting and then start working on the frontline if you cant reach the backline. Dont try to Yolo in. If you have like a nautlus or a malphite in the enemy team, you have to be really careful with your positioning.

In a "Hyper Carry Comp" (When you have like a lulu/janna/... in your team) you can play more aggresive and try to poke the enemys. But you should remember that if you your team has no big chance on winning the Teamfight.