I found that I have to start the VPN in the client first and I have to share the Airport (or Ethernet) connection with the Parallels (en2) connection in OS X. Once the client VPN is up, I can get the host VPN to come up and both work in parallel (no pun intended). If, however, I start the host VPN first, the target domain cannot be resolved in the guest, therefore the VPN cannot be established.