Yeah I already tried that. I went to file associations in Foobar first and made sure it was changed, then went to file associations in vista itself and made sure it lists default program for .mp3 as Foobar 2000.
The thing is, it IS the default, until you select more than 15 files at once. As soon as you do that, the button changes from saying "Play in Foobar 2000" to "Play in Windows Media Player" as if Vista doesn't think anything else but media player could actually handle opening more than 15 files at once.
Its very odd.