If I play a Bureaucrat and my opponent reveals a hand with a Curse card, does that Curse card go back on top of his/her deck? Or does it not count as a "Victory card"?
No. Curse cards do not have the type Victory. They just have type Curse.
(You can see this in the rules or by reading the card closely - although they affect your VP total, they do not have "Victory" in the type line, just "Curse").
On page 2-3 of the rules you'll see the various types of cards.
Curse is it's own type of card and not a Victory card. On page 2, the explanation of the Curse card includes "card type, (curse, purple)