I've had someone tell me that the vampires arm would mist and return to the coffin. I disagreed because that should only happen when the vampire is reduced to 0 hit points.
A 3.x vampire can turn to gaseous form as a standard action.
Allow the gas of the limb to rejoin the rest of the body, and reform as solid. Poof, one restored limb. He doesn't get any hit points back in this process, mind you, but he doesn't have to hop around like a ninny on one leg either.
I wouldn't call a sundered limb necessarily cut off, by the way. It would be damaged beyond use (say, all the bones shattered so well it just flops there, or muscles or ligaments severed), but that's different from outright dismemberment.