unavoidable attacks are specifically designed to be survivable without the extra vit, though (assuming average gear, if you're in hardcore progression then it becomes an entirely different conversation involving crafted gear). In theory, sure, but the number of situations where an unavoidable attack will knock a non-tank under 450 without killing them are effectively 0. That's another important part: you're only giving yourself a 450 hp window, which is very specific. The odds of you getting overkilled by something within that margin is very small.
So if you feel you've run into a specific situation (like gigaflare or terraflare during progression or something) then I wouldn't judge someone for switching to vit, even though I'd go a different route myself. But if we're still talking about general situations, then the extra main stat will be useful far more often than the health.