Using Zeroshell you can create a lan-to-lan VPN for each WAN link you have and then you can create a BOND virtual interface configured in Fault Tolerance or Load Balancing. In both the cases, if one of the VPNs goes down the traffic is automatically re-routed on the other ones. If you use the load balancing configuration you can increase the throughput of the virtual site-to-site connection proportionally to the number of yours WAN links.
For more information look at
http://www.zeroshell.net/eng/faq/vpn/#vpn.faq3
http://www.zeroshell.net/eng/forum/viewtopic.php?t=104
http://www.zeroshell.net/eng/forum/viewtopic.php?t=111
Regards
Fulvio