The ways to counter Vi's Ultimate
 

Let me start of by saying this: for the way Vi's ultimate is presented, it is supposed to be this un-counterable attack that singles out an enemy for your team to destroy, but there are several ways to escape from it.
The first way, is Kassadin's Void step. If you void step away when Vi takes you into the air, you can escape unscathed. It may be possible to do this with flash, shunpo and maybe even kha'zix's jump-thing.
The second way isn't really a way to escape from it but; you can use Darius's ult during the throw part of Vi's ultimate and land it before her ult finishes.

Since when is it unscathed? Every time I saw someone flash out of it they were just slammed into the ground away from Vi.

Vi? You can counter MF's ult by sneezing in her general direction....

From my experience, Kassadin stepped over a wall and out of the fight, so I didn't see what happened after the fact.

Unrelated.

Can't you just flash over a wall? Pop an hourglass? Blitz grab her? Hec/Noc fear her? I'm just asking. I have played her many times and have seen what her ult can do and yes it is devastating but from what I can think of it's stoppable no?

Right, thats my beef: it is advertised as unstoppable once cast, but it can be stopped or avoided in several ways.

Ah ok I see what you're saying now. I think Riot should just reword it. I don't think it should be "unstoppable" but if use properly it kinda is.

True, but on the other side of the coin, it is perfectly avoidable/stoppable if you attain the correct circumstances (vlad's blood pool, for instance)

Olaf ulti pls