VPN & Proxy

I have a client who is running Windows NT 4.0 SP6 with Proxy Server 2.0. The workstations are running Windows 98SE. The Win98 workstation are currently accessing the internet through the proxy server using a dialup connection. My client has to now perform subcontracting work with a 3rd party. Access to the 3rd party network is provided by VPN. Can anyone help me with a solutions that would allow VPN/PPTP traffic from the 98 workstation across Microsoft Proxy Server 2.0. The workstations have connect to the 3rd party network using VPN. once connected to the 3rd party network they will be accessing a unix application using telnet. Any help would be greatly appreciated.