I can confirm what Slats says about that group setup. My second set of 70s were 4x shadow priests that I played with a prot pally buddy of mine on a PVE server. We were fine as long as we outgeared/outleveled the instance. Once we got into spots where stuff hit harder than VE/VT could keep up, things got dicey. So, basically, what we usually did was stayed shadowform and just healed him through VT/VE/flay spam until we got to a large pull or boss mob. Then I'd switch out and drop VT/VE/SWP and keep renews going on him. That was enough to do very well through most encounters and the VT x4 spam kept him with enough mana that he could spam everything he had for aggro management. That part of things rocked. The sucky part was that if things got ugly (like trying to do bog lords in UB at 64...those hit HARD at that level) then I'd usually end up completely out of mana from trying to maintain DPS on the target AND keep him healed.

If you're really interested in still doing it, let me know and I'll go find some of the videos of us doing instances and world boss mobs like the Gorefiend's Armor dragon, Uvuros and similar.