You would accomplish it using a player like Foobar 2000 and the WASAPI output, with exclusive control enabled. That equates to a bit perfect output. There's no way to accomplish the same thing using iTunes with Vista, as far as I know.
I don't believe that Vista uses Kmixer, but rather an improved mixer that's totally different than how XP worked. ASIO is also not something you really use in Vista to bypass the mixer, it's now referred to as WASAPI.
FWIW, I can't really hear a difference between the "bit perfect" Foobar 2000 WASAPI output and iTunes through Vista. If there's degradation in the iTunes output through Vista, either my system is not revealing enough or my ears aren't good enough.
I don't believe that Vista uses Kmixer, but rather an improved mixer that's totally different than how XP worked. ASIO is also not something you really use in Vista to bypass the mixer, it's now referred to as WASAPI.
FWIW, I can't really hear a difference between the "bit perfect" Foobar 2000 WASAPI output and iTunes through Vista. If there's degradation in the iTunes output through Vista, either my system is not revealing enough or my ears aren't good enough.