While adding vCenter / ESX host, OpManager discovers all the VMs during the initial discovery and the specific VM's to be monitored can be selected and discovered.
If a VM is already being monitored by OpManager, it can be deleted from the list and it will not be added again.
Click on the relevant icon to monitor the required VMs on the host. OpManager maintains this configuration when devices are rediscovered. (HA/VMotion)