Actually, it doesn't return the same amount of mana. If you were clever about how you applied VT and used downranked DoTs and flay, you could chain up mana-positive rotations on mobs. Pre-3.0.2 I'd take 3 of the Wretched Fiend/Devouers in Q'D Isle on at once and be close to 100% mana once I was done.

The thing is, before downranking was removed you could do a lot with VT, now you can't. In fact casting VT+MB now will always cost more mana than you get back from Replenishment, even if you are a level 80 Spriest in 25-man epics. It is a group utility spell, it isn't there to replenish your own mana. If you want to replenish your own mana while casting, unfortunately your two choices are:
1. buy up Imp Spirit Tap and crit a lot (RNG dependant)
2. buy up Meditation in Discipline (lose Shadow points)
3. stop casting and wand (cruddy DPS for a DPS-only spec)
4. pop Dispersion (stop DPS for a DPS-only spec)

or.. secret option #5, reroll a ret pally since even with the 'nerfs' they can, with mixed epics, blues & greens, put out more damage than an epicced-out Spriest in 3.0.2 while dropping their mana bar no lower than 85%.