I was not able to figure out how to direct each vpn tunnel to their own gateway without using the netbalancer’s gateways, like Hibbelharry mentioned. I don’t think static routes will work because we are trying to get away with only 1 public IP at the data center. Otherwise, with enough public IPs at the data center, a static route for each vpn tunnel’s target IP could be routed to the corresponding local internet gateway.
In my case, when I first tried this I was bridging the vpn bond on my local zeroshell with it’s eth00 interface and this allowed me to specify the data center zeroshell’s vpn bond IP as a gateway. It kind of worked.