If you have life magic trained, you should get the lvl 5 or 6 spells at least, so even if debuffed and out of stam items VTank has a way to restam. Level 5 are not that expensive and still work ok in many situations.
As for the buffing while being under attack: VTank keeps track of buffs and recasts them before they expire - with a higher priority than fighting. This makes sense, after all you don't wont your banes or prots to drop while you are surrounded by a dozen foes. To decrease the likelyhood that you need to rebuff while under attack it is best to turn on "Rebuff when Idle" - in which case VTank will rebuff a bit ahead of time if it has nothing else to do (nothing to fight, loot or salvage).
(And follow Agnari's advice below. Most of my melees have higher effective melee when buffing than during combat because they all have brassed up wands - I often run around with my wand out instead of weapon and shield if I just want to run to the end of a dungeon or something.)
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