Microsoft Network Client Login with VPN over DSL Question

I was using the Nortel Contivity VPN with DUN, and it brought up a
Microsoft Network login screen after the VPN connection was
established.

Now I am using Nortel with DSL. Is there anyway to trigger a network
login screen when establishing the connection to the VPN? Can you
make a dummy DUN connection somehow?

I know that I can accomplish the login by setting up the Microsoft
Network Client as my primary network login, but I would rather do it
as needed when I use the VPN.