Extranet, Vista & Dlink

I use the Nortel Extranet VPN client to connect to work.
My Internet connection is wireless via DLink or USR router/Access Point.
I have a laptop running Windows XP and a desktop running Vista.
The laptop VPN client connects to work fine through either the DLink or USR.
The desktop VPN client only connects via USR (via DLink does not fine the
Extranet switch at work).
The difference between the 2 routers are:
DLink is an unsecured network with network address 192.168.0.x.
USR is a WEP-secured network with network address 192.168.123.x.
When the USR is made unsecured the VPN still connects fine.
Because the laptop connects fine with either one then the problem is
unlikely to be on the VPN client and because the Vista VPN connects fine
with unsecured and secured USR then the problem is unlikely to be on the
routers.
May it be possible that the problem is the network address 192.168.0.x?
If so, how could Vista be configured to allow this address to work in VPN?
(Does not have access to Dlink configuration)