setting up emac-vlan

Hi, theres very little information about emac-vlan setups till now, cant find any video on youtube.

I would like to implement this feature on a running HA Fortigate setup. My concerns are with converting existing interfaces into emac-vlan enabled ones. I have a typical router on a stick config, with one subnet connecting interfaces from LAN RTR:

WAN RTR

|

Forti

|

LAN RTR

I would like to add some vdoms. Do i need to delete existing interfaces, and create them as new emac-vlan int. on root vdom, before i can interconnect them to new vdoms ? On WAN side IP is set on physical int, on LAN side it is a vlan-interafce.