Just as the pool discovery has item prototypes for pool status enabled/available, the virtual server discovery rule should have item prototypes for the same.

Virtual server status avail oid: .1.3.6.1.4.1.3375.2.2.10.13.2.1.2.{#SNMPINDEX}
Virtual server status enabled oid: .1.3.6.1.4.1.3375.2.2.10.13.2.1.3.{#SNMPINDEX}

Corresponding triggers to match the pools triggers should also be enabled. The Virtual Server availability trigger would ideally be a disaster as it would indicate full unavailability of a service, with scope set to availability (presumably).