I reckon the Vaccinator is currently viable in every situation bar the midfight, but I imagine it could be made to work there. The biggest problem right now as I see it is that people (non-medics) don't understand the mechanics. They just panic because they don't have a buff even though they have a 10 or 75% resistance. They also don't build at max rate either (you need to be lower than your base health).
An oversimplification would be that 1v1 a standard full buff is essentially a 50% resistance. Vaccinator is giving you 2-4-6-8 seconds of 75% resistance (if you pick the right target/fight).
Equally a big part of the Vaccinators usefulness right now is that people don't know how to play against it either. It's very easy to tank through an unsuspecting demos sticky trap or bum rush a complacent soldier holding a choke point.
Best way to beat it is to blob up with multiple damage types. Good pyro in the mix actually pretty annoying to play against with Vaccinator.
It is also the best weapon hands down for medic survivability. Roamer bombs you? Bubble. Scout rushes you? Bubble. Sniper in the distance? Bubble. Don't have a bubble? 1 Ubersaw = 1 Bubble. This weapon is super clutch. With half a brain and decent positioning you can kite and survive any enemy charge.
Anyone tested how much vacc charge you get from a max heal crossbow?
I tried using it in a highlander scrim once, but wasn't well received. The world isn't ready yet for my vacc plays :D
I maintain that if a team is smart and play aggressively, isolating enemy players by picking fights with the right resistances the Vaccinator is viable in comp play.