I connect to a third party service from my office, where we have a
Cisco router which offers NAT addresses internally. The third party
knows our externally facing IP address and is configured on their end
to accept connections from it and everything works fine.

When I am on the road, I sometimes need to use a dial up ISP to get on
the internet, and then connect to my office using a VPN (PPTP to a
Win2k server with a static IP in my office). This works fine, however,
I cannot use the application that connects to this third party. How
can I force the traffic to go through the dial up ISP, over the VPN to
my office and then out to the third party so they will see it coming
from my office's public IP?

I tried entering a static route as follows:

Route add mask 255.255.255.255 assinged by W2K>

When I do a tracert to the third party, I only get asterisks back.