Well all of the Leporacanthicus genus are considered vampires, but Leporacanthicus galaxias is reported to get 25cm/10", but personally I've never seen one reach this size or be imported quite so big.
That pleco does not eat algae however but does enjoy wood and meaty foods and veggies. There are quite a few plecos that only get a few inches. I have a queen arabesque in my planted tank but they are not much of algae eaters either. It will eat alittle but prefers to munch on wood and meaty foods also.