Interesting. You can try to give your guest a second network adapter that's "host-only". Then try whether guest and host can ping each other over this network while the VPN is off. Then fire up your VPN client, make sure it uses the first network, log in to the VPN and try the ping on that second network again. It might not work anymore because VPN clients tend to disable all networks they find on the machine. There might be a workaround for your particular VPN client, but I have no experience with that. The ones I've tried don't allow any network besides the VPN.
What works, however is folder sharing. I'm using that every day. I have a VM that connects to a VPN, downloads emails, dumps them into a shared folder, and I read them from the host. Maybe folder sharing is a compromise for you as well. The nice thing for me is that I need the VPN only for email and with this setup, the guest holds the VPN connection for only that purpose and I can still use the regular Internet on the host.
