VBR on a dedicated network adapter?

If the target hosts are on a specific subnet and the Vertica nodes have an interface configured in that subnet it will just work. If there are more devices in between you just need to play with the routing table, you can validate this by using ping with a parameter to specify the interface you want to use. (I think it's -u but not 100% sure)

yep, as long as there is a single route to the backup location via the adapter you chose the kernel will route all packets over that card, effectively giving you the desired effect. use route command to set the route and tracert to check its what you want it to be.

Thank you!