He didn’t know there was vampires as it said they were once(?) human so he didn’t think anything of it he just wanted to save people
well. maybe that's the case (~‾▿‾)~
To add to that, normally he would add a bunch of people at once (people tend to come in packs) so he wouldn't have the time to go through every single person one at a time to scan for vampires. PLUS, they can turn into one after they joined too so rescanning every single person is too time consuming given his packed schedule
And then after finding out that he messed up big time by letting a vampire in and killing the citizens, he became extra careful with whom he let in.
He was basically juggling efficiency with solving the problem the best he could.
Hence, the solution he came up with (づ￣ ³￣)づ
didn't MC have the skill to remove citizenship before, why not do the same to vampires?