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Check if there wireless/ physical adapter disables IPV6. It is likely that, when you disconnect from the VPN it tries to use IPV6 instead of IPV4.
One more thought, if the vpn is using the default gateway on the network, it may be causing for disconnection. Not sure exactly where in the connection properties its listed but it may be causing glitch in the default gateway assigned to this IP.
Or you've got something installed there (just guess) which is causing the issue.