I'm looking at both of these phones as well. They're very close spec-wise, according to rumors.
If you want a pure ICS experience sooner rather than later, I'd say wait for Prime. But I'm pretty sure Vigor will get ICS ported over by devs eventually, and it most likely will get an official update even later.
The only thing that's got me leaning towards waiting is HTC battery life history, and the fact that ICS will do away with capacitive buttons, and the Vigor will be stuck with those.
But even if you get the Vigor and have to deal with GB for a while, it's a very stable and smooth interface at this point. HTC Sense 3.5 seems to be very nice, too.