You say you want a SIP phone but via USB. All of the SIP phones I have EVER seen are proper stand-alone IP phones connected via CAT5 since SIP is the high-level communications protocol. So the issue of Vista drivers is non-existent [that's the whole point of SIP].
You could get a SIP softphone and use a mic/headset combination or see if one of the softphones supports one of the USB phones as input/output.