Well, I've got a mic working just fine in Vista-64 bit with just plugging it in and starting up the vista voice recognition and microphone setup stuff. Unfortunately there isn't any real way of knowing why a piece of hardware doesn't work on Vista as the drivers are just badly written most of the time. If you go into Control Panel->Sound->Recording and don't see your microphone on the list then it's not plugged in, has a bad connection, or any number of driver related things could be wrong. If it's finding the microphone and it's just hal not working then things get trickier. I find that I have to start up Vista voice recognition and then set it to off so it doesn't try to run any of what I say as commands to it. Just tell me any further information you can provide and I may be able to help out better (ie. microphone being used, are drivers installed for it, all that nonsense).
EDIT: Also be sure when your in the sound settings that your microphone is selected as the default microphone. There should be a green checkmark next to it, and if there isn't right click on the entry and you should see an option to enable it or make it default.
Also if you don't see the device and it's plugged in you can try right clicking in the area where the recording devices are listed and selecting the option "show disabled devices," as this will allow you to see any devices you may have inadvertently disabled so you can enable it.