Although Stretch was filled with goo that allowed him to perform his Plastic Man tricks, Vac-Man was filled with grainy pieces of "vegetable matter." Once you twisted his body into some malformed shape, you could shove the Vac-Pump into his ear and suck all the air out of him, and he'd stay in whatever shape you'd posed him into until you let the air come whooshing back by pressing his "cybernetic button." It didn't take long before you'd put him in the most perverted position possible and leave him like that until your mother confiscated the toy. Not that we'd know anything about that.
Check out the extremely informative instructions for Vac-Man in the gallery. The best part is that if Vac-Man breaks, don't despair! You can get a parent to fix him with a balloon, some rubber cement, and a little know-how. Just try doing that with a damn Xbox.