If it is just the one server you could just use PPTP or IPSec dial in and assign it the existing IP address from the router. This would mean a bit more work for the server but it is nice and simple.
Another way is translating the network which is still fairly simple.
Or you could make the single IP of the server another subnet in the VPN and use LAN-LAN
or you could just assign a different subnet LAN to LAN and change the DNS or WINS lookup to point to the new IP.