Test connection
Only the master server needs to be added and If it gives error for ports and ips while clicking on test connection button on WHMCS then you will need to add required TCP in-out ports (4081-4085) in WHMCS firewall.
To further troubleshoot the error you can also try to curl from WHMCS ssh : curl -kv “https://virt-master:4085” and from Virtualizor master ssh : curl -kv “https:///path/to/WHMCS/modules/servers/virtualizor/callback.php”.
Note : If its on shared hosting then mostly they won’t allow the required ports.
Plans load issue for product
You can try clicking on save once in module settings and if it doesn’t helps then make sure correct domain is added for Virtualizor server on WHMCS panel in hostname field.
Else you can leave the hostname field empty and save it and then save the product settings and check if plans load or not.
Vnc issue on WHMCS
If WHMCS is being accessed on SSL then Virtualizor panel also needs to have ssl and also make sure correct hostname/domain is added for Virtualizor server on WHMCS panel.
Accessing panel over domain will require 'use server hostname' in Configuration->Slave settings.
Before trying vnc on WHMCS, also verify if vnc works on Virtualizor Admin panel over domain with SSL.
If the vps is on slave then make sure you have selected 'use Master proxy' in Configuration->Master settings while accessing vnc from Master panel.