...I feel like I obviously haven't made something clear. I'm assuming 252 HP / SpAtk with both Assault Vest and Leftovers, and saying that with this spread, Leftovers may well be superior in the common scenarios mentioned (specifically, playing against double switching or weak attacks such as Volt Switch with Stealth Rock / other hazards up). The Leftovers spread may benefit more from some added special defense investment to avoid certain KO's, but it's irrelevant to the current discussion.
Ulevo, surely you can understand that calculations aren't necessary to see that Leftovers could be superior? If you're taking hazard damage or minor levels of damage (such that the damage reduced by Assault Vest is less than what Leftovers heals), Leftovers is more effective; if you're taking predominantly heavy attack damage, Assault Vest is better. This is fact. At some point on this scale (hazards/minor attacks/status damage vs heavy attack damage), Leftovers and Assault Vest are equally useful. I just don't understand how you can possibly dispute these conclusions?