Its a problem with the design. I've played other RPGs where the inventory was realtime (Diablo :shrug:; Arx Fatalis, Baldur's Gate, Eye of the Beholder); I've also played RPG's that specifically prohibit changing armor in combat. The original Fallout charged AP's to enter inventory while in combat (but once there, it was just as bad as FO3 or Oblivion... You could use as many stims/potions as needed to heal completely and even change armor IIRC).
TES/FO3 has the engine set so that its either in game (real time) or in-menu (everything stops).
I don't equate VATS with the inventory though... Inventory lets you use whatever you have to heal, buff, and alter your defensive and offensive ability instantly; Vats just pauses the game while you pick your target. What VATS does that FO1&2 did not (not in the same way), was let you cue up shots. Technically you do this in Fallout 1 every time its your turn, but the AP system is different and the AP costs are different, and afterward everyone else does it to you. Fallout 1 played by double edged rules; Fallout 3 plays favorites to the PC, and gives unlimited ammo to the NPC attackers. Its royally screwed up. Its not something I'd like to see done in Skyrim or other future TES games.
TES combat was always the FPP simulation, "be there, live it" kind of experience; an attempt at realism. The VATS idea would pull you out of it, and has no history in the series anyway.
**Moot point to me though :shrug:, I almost never play an RPG as though living it, or being there... I play almost all of them as an observer of the PC in their own environment.
*** I expect more of the same, but with better textures ~meshwise, its still has to run on an Xbox, :shrug:
(I'd love to see them incorporate the spell casting method from Arx Fatalis ~but I don't expect them to do it.)