Firstly, please don't underestimate the size of their mouths. That has nothing to do with the damage these cats can inflict. A well fed vulture will generally not touch healthy fish, but in my experience a very small abrasion on an otherwise healthy fish can be a great incentive for a fish who is on day six of a weekly feeding pattern.
I had my vulture take some nice bites out of my 2.5 ft pengassius when he had a small abrasion and they were not all in the same place , and then he went on to remove a 2" by 8" strip of flesh off him under his dorsal fin, "just because he could". And leave it for other fish to eat!
I love the fish, but I don't trust him or leave him more than three-day without good anymore.
As for the question of other alternatives if an 18" to 24" vulture is too big, yes there is a perfect other option for a 6 ft by 2ft tank or bigger. A bolt catfish, aguarunichthys torosus. All being well i will be setting up a thread on this fish shortly, but is great similar in many respects but the bolt only grows to 14" max.
Note however, that I would also suggest that a bigger tank is always preferable.