If the 500 year old vampire child is with a regular human, the attraction is quite clear on the human side of things and that person is obviously a pedo on SOME level. They're just finding a loophole around the extra "sick fuck" factor. Whether the 500 year old vamp child gives a shit or not is up to them. (Probably not)
This of course is not taking into account the whole hypnotize thing vamps do. If the 500 year old child vamp is doing that to get laid, then the human wouldn't be a pedo, but would in fact just be a rape victim.
A 500 year old child vamp fucking a "regular" vamp would still classify the adult vamp as a pedo, but again since they're both unnatural creatures anyway, I doubt if either gives a shit.
Where things get interesting is if the 500 year old kid vampire fucks an actual human kid. Though I'd argue that's still being a pedo, just in this case its the vampire.
500 year old child vampires fucking similar 500 year old child vampires is the only way sex stuff can happen without either party being a degenerate.
Midgets are just folks with stunted growth. Same logic doesn't apply as long as they're of age like a regular person.