VPN Connection Troubles

I have configured my Windows 2003 Server box to allow VPN connections. Once
the VPN connection is established, users start Remote Desktop Connection and
they can then work as if they were sitting at their desk.

I have had absolutely no problems with this, except that now one of my users
has Windows Vista. While they can establish a VPN connection, they cannot
RDC into their computer. I get an error if I use a computer name, and I get
the same error if I type in the IP address.

Near as I can tell, the problem is the subnet mask. On a Windows XP
computer, my DHCP server hands out an IP address like 10.0.0.119, a subnet
mask of 255.255.255.0 and DNS Server of 10.0.0.5. All of this is correct.

For whatever reason, on the Windows Vista computer, the DHCP server hands
out an address like 10.0.0.119 (good), a subnet mask of 255.255.255.255 (not
good) and a DNS Server of 10.0.0.5 (good).

So whats up with the different subnet mask on the Vista computer vs. the XP
computer? I should point out that after fooling around with this, I now have
whopping 30 minutes experience with Windows Vista, so please, please, please
be very specific if you can help me.

Thanks!
Brian