Real question, I was getting one hit in 0.17 seconds the other game as a vayne with no way of stopping it as his ult was up every time I'd respawn. So genuinely, how do I counter this without having to be grouped in 5 of my teammates (as, at least with this game, they were not cooperating)?
You need a teamcomp that doesn't allow him to just freely jump on you. So- any quick CC tank/support that can immediately stop him from killing you, or worse case scenario, immediately murder him after he murders you. Isolated ADCs are supposed to be free kills for assassins.
You don't.
It's hard to deal with him.
for sure deny a lot of his oneshot potential (but it's not enough if you're at least 2 lv behind)
probably it's the only choice but when you manage to get this, he should be full item.
Without this item maybe only a fast reacted 
E can save you.
I would count 
cc but probably end up oneshot you anyway without defense,
mobility but he still need tank item,
shield but she would got oneshot before to use it.
Other counterplay require at least tons of defense item or a defensive support.
Honestly sometimes I straight up build a randuin it's really nice item for ADC vs ADC fights and helps a ton vs any assassins. Ga can be okay but as a vayne you don't want to be popped you want to survive. You can go ultra defensive with death dance and phantom dancer too.
Ask your support to stick with you and he won't kill you
