we had a similar problem on one of our 2008R2 servers that transmits credit card info to our clearing house.
we connect through VPN, and the remote network was not trusted, so all domain-based networking services on our server would become unavailable as long as that connection was active. it was only for a minute at a time or so, but it was enough to kill clients attached to the order processing software.
ultimately, we solved it by going into windows firewall with advanced security (in administrative tools, not in general control panel) and setting up custom inbound and outbound rules that treated the vpn connection as a trusted one.
only other thing that jumps out is if you have your vpn profile set to route all traffic through that connection when connected.
dunno if this is what's going on with you, but hope it helps.