It is very strange. Could you try to encapsulate the BOND00 interface in a BRIDGE00 one and enable the dhcp client on this?
I tested (beta11) the VPN bonding only in failover and load balancing and it worked fine. The modality “only failover” could not work correctly. In this case I will fix it for the next release. In any case, VPN bonding is necessary only to increase the layer 2 traffic among remote offices by using the load balancing. If you just need of a VPN with fault tolerance the bonding is not necessary. You just need to configure the Net Balancer with the multi-gateways and then select the preferred link in the site-to-site VPN configuration.