Tried unplugging the cable until VPN comes back up?
In any case you could delete all the Wired/Wireless connections that Network Manager has stored and just leave the VPN ones there with the "automatic" option activated so when VPN comes back up it will connect to it automatically.
Just to make my point a little more clear. In the Wired, Wireless, Mobile Broadband and DSL set any connections you might have there to not automatic (Remove the Connect Automatically check from the checkbox). Or to be more drastic remove the connection completely. The only one that should be in Connect Automatically is the VPN connection you have.
With this you should have a system that automatically connects to the VPN when it is available. When it disconnects the system will stay offline until the VPN comes back.