I have a Netgear RangeMax wireless router. Whenever I connect to this router either wirelessly or via an ethernet cable and try to use VPN, at some point during the VPN connection all computers connected to the router will be forcibly disconnected, seemingly at random. Furthermore, while the computer using the VPN is connected, there are "mini outages." This machine is Windows 7, and every few seconds the Internet Access icon in the tray showing that I'm connected by an ethernet cord will get the yellow exclamation point and say there's no internet access, but the outage isn't long enough to cause the VPN connected to break. Eventually, as I mentioned above, the connected does break, though. What's the deal? Time for a new wireless router? When I plug the computer in directly to the modem (circumvent the wireless router), I have no problems. So I've identified the wireless router as the problem.
I had a similar problem with my corporate VPN and a LinkSys router -- I tracked it down to UDP traffic when the VPN connection was created just being far more traffic than the router could handle. I moved to a D-Link DIR-655 and the problem went away.