Something to try, manually define some DNS servers on your local PC(not dhcp)
in the vpn configuration 'Click Advanced and uncheck the box for "Use default gateway on remote network." This will route all of your local traffic through whatever network you're locally connected to, and any remote traffic through the VPN connection. '
then when the vpn comes up it should add the remote DNS server to the END of the list...vpn to resources may take a while to resolve, but it should work. if it still kills your dns, disable dhcp configuration of dns server and set the vpn network dns server manually and at the end of the list
trying to answer another question I ran across this: http://technet.microsoft.com/ru-ru/library/cc786865%28WS.10%29.aspx
the article speculates that you can attach post connect scripts. so you would need to do that and have the post connect script modify your DNS entries.