How does the built-in Vertica load balancer work with sub-clusters?

Load-balancing with Subclusters works very similar to load-balancing with Fault Froups. When load balance group is created using the 'WITH SUBCLUSTER ' argument, the nodes in that Subcluster will participate in the process. And yes, round-robin is the default mode, but can be changed if desired. Please note, that as with Fault Groups, you must create network addresses on the nodes in the subcluster that you want to be part of the load balance group.
Here is the example of the syntax:

CREATE LOAD BALANCE GROUP load_subcluster WITH SUBCLUSTER load_subcluster FILTER '0.0.0.0/0';

Please see more in Vertica Documentation: https://www.vertica.com/docs/9.3.x/HTML/Content/Authoring/AdministratorsGuide/ManagingClientConnections/LoadBalancing/CreatingLoadBalancingGroups.htm?zoom_highlight=create load balance group