The 'vamp' in Buffy's house--
What no one seems to be considering here is that this wasn't a vampire... or at least not what commonly passes for a vampire in the Buffyverse... it's someone wearing a vampire costume. From Ethan's, I mean.
So, imagine earlier that day, some poor guy walks into Ethan's costume shop, trying to find a costume because his girlfriend is badgering him into going to a party that night and costumes are mandatory. He has no idea what to get, but knows that said girlfriend, like many, many other females, are really into vampires. He wanders the shop, and what do you know, they have an entire AISLE of vampire costumes.
Louis or Lestat, from the Anne Rice books.
Damon or Stefan from 'The Vampire Diaries' (the books, which were out at the time, not the series, which was not).
Basically what I'm suggesting is that whichever costume he bought, from whichever setting that vampire inhabits, is what that guy became when the spell took effect. Therefore, 'Random Guy Vampire' became a Vampire who followed THOSE RULES (Including the Invite requirement) and NOT the Buffyverse rules.
Louis, for example, would be a vampire, but would not be affected by holy water, nor would he require an invitation into the house. (He would, however, be really depressed and whiny ^_^)
And in passing, I've always wondered what the body count was for the whole Halloween thing. I, for one, very much doubt that Soldier!Xander was shooting to miss in every instance of gunfire, given that he was seeing hideous creatures attacking, wounding, and probably killing normal humans.
And there were a LOT of little monsters running around with no Slayer to stop them, who were very intent of hurting/killing humans. You have to figure that a fair number of them succeeded.