I have the same problem at a number of clients and the same problem as others at Experts Exchange which have not got a solution. VPN is working with simple pptp. I can browse etc.
NSLOOKUP shows ISP DNS servers before Internal DNS servers for the clients; that looks wrong to me.
IPconfig /all shows properly for WINS and DNS all showing internal IPs.
DNS works properly within the network for everyone internal. Exchange is at abc.def.com internally at 192.168.1.1 and our ISP has an identical host name abc.def.com registered for an external address of xxx.yyy.zzz.aaa which also works for VPN connection. The only time it doesn't work is after a vpn client accesses the local network for exchange and gets blocked because he gets the external ip for the host name. I tried lmhosts lookup in the VPN networking advanced properties and also unclicking the "use default gateway on remote network" I put static entry's for DNS and WINS which did help for some things but not this.