A vampire feeds not for sustainable but to become more powerful and regenerate wounds. In terms of longevity there is no difference between a fully fed vampire and a vampire that hasnt fed in a millennia. They are reservoirs not engines: They exhaust blood when they need to.
Despite this, the vampire is constantly haunted by every single person they drank to death. The only way to silence the specters of their sins is to drink more blood, imparting a soporific effect that while not completely drowning out the screams, makes them far more quiet.