The administrator account is not active by default. You need to activate the account and set a password for it from vista. You can do this from control panel/administrative tools/computer management. From there you can open the tree view under users and you should see administartor, right click and select properties. You can then enable it. Then add a password in the usual way, and use these to login through the vmware web interface.
I found out, that in Vista, using net user Administrator /active:yes and I after logging and changing the password, I can get into the Server Web UI.regards,