You may have problems. I had a Tesco VoIP cordless handset that stopped working when I changed to Vista. The reason was the firmware in the Tesco phone was only suitable for XP and wasn't upgradable. For Vista Tesco have a newer version that will work with Vista. They allowed me the original purchase price of my XP phone against a Vista ready alternative which meant a straight swop as the prices were the same.
Since then I have added an ethernet adaptor that works when the router is 'on' but the PC is 'off'. This has the advantage of been independent of the computers OS and allows any standard handset to be used for VoIP. Works very well.