Office Communicator 2005 is most definately compatible with Vista. Just make sure your version of vista is up to date. And make sure you patch Office Communicator 2005.
It is also compatible with Office 2007 as well. I am running Office Communicator 2005 on about 50 Vista machines with Office 2007.
Side note:
Office Communicator 2005 will not work with LCS 2007. If you have LCS 2007 you need Office Communicator 2007.