Unless I am mistaken, Angelique consistently sees her reflection in that mirror. While it's likely the writers simply forgot the mirror rule or took artistic liberties and dispensed with it for the sake of the story, there are two other possible explanations:
As we have seen, it's no ordinary mirror; it's a magic one. Perhaps whatever spell is on the mirror allows even vamps to see their reflections.
Either that or the reflection rule doesn't apply to Angelique because she's no ordinary vampire; she's a Vampire Witch.