Vista should not prevent you from using the full functionality of Outlook Web Access, and the fact that you can use OWA on non-Vista systems indicates that the problem is not at the server.
The features that have the problems (reply, forward and compose) all involve OWA generating popup windows. Therefore, I would check your Vista for any popup blockers. Add your OWA site to the list of “allowed” sites, and perhaps also to your browser’s trusted sites list through Tools > Internet Options.
This issue could be the client computer or the exchange server. Here is a good discussion that may have the answer. Take a look. Much good info is shared.