You could read the tutorials page, there is a tutorial on bonding vpn tunnels together and thus you could save a lot of time.
http://www.linuxplanet.com/linuxplanet/tutorials/6799/1/
When you create a VPN tunnel you can assign which gateway to use, so that you don’t need to worry about having netbalancer or any other static route working. Once the tunnels are up and BOND is working you can use it for default gateway.