Did you update your drivers at the Creative site? Using the ones that came with the card rarely work with Vista. Hell, the ones on the disc with the card rarely work at all.
And I wasn't kidding about the rapid plugging and unplugging and voodoo. Well, maybe about the voodoo bit, but it can't hurt. Many X-Fi cards have a "universal jack" on the card that's supposed to allow it to adapt to what gets plugged in, when it's usually just used for a microphone. This jack almost never works properly in any case I've had to deal with one of these damn cards, and fixing the problem's a crapshoot. Only thing that's ever worked for anyone I've troubleshot the issue for is to make sure all the related drivers are up to date, disable on-board sound, and then repeatedly plug the mic in, test it, unplug it, wait, plug it back in, test it, etc. Eventually it'll work, but it's not guaranteed to last.
Sound cards suck these days. You used to be able to buy a Creative card without issue, but when even their $150 sound cards have issues like this, I switched to on-board sound instead of replacing the Creative card that died on me.