Cool! Vampire's (Leporacanthicus galaxias) grow to be about 8-12 inches, so he should be find in the 55, at least for a while. They're not true vegetarians, so I'd supplement any algae wafers you might be feeding him with, with some cory food or something like that as well (they love shrimp). They don't seem particularly aggressive, and get along fine with other bottom dwellers, as long as it's not too crowded. Watch out though, they can bite you! They have (I'm not totally clear on this) these little fang-like things that protrude from their mouth.
It's a good accidental purchase anyway, even if you decide not to keep him. I've seen them for sale for anywhere between $30-65 US. Quite the deal if you paid the price of a common pleco!