The movie IS canon.
When Buffy is first checking into Sunnyvale...they mention she burned down her last schools gym and she mentioned it was full of Vamp...
But that actually contradicts the movie, since the gym didn't burn down in the filmed version. There are a number of ways in which the film and the series are contradictory. Here's a list:
http://inner-moppet.net/hemery/differences.html
So no, the movie's version of events is not canonical in the series' universe. Rather, the series incorporates elements of Whedon's original unfilmed script, which told the same basic story in a different way. Dark Horse Comics did an adaptation of Whedon's script, which is pretty much what Buffy's origin would've been like in the series canon.