Yeah, I noticed that with Myles, who had a lot more hitpoints outside the party than in
But I don't see how Vi could be killed. She's a Valk, after all, and they can cheat death.
In any case, no, you don't actually need her in your party at any time, including Ascension Peak. Despite what you hear in the game, she isn't necessary. The designers may have had it that way originally, and changed their minds (good thing, too).
Still, it's odd she wasn't there for the rescue. Never had that happen to me. Something to do with the patch, perhaps?
Nightowl2