VDOM or Zone??

Hi all, 

First fortigate, first time setting up VLANs and VOIP so absolutely in the dark sorry!

We have a fortigate 50E

Two incoming WANS 1/ ISP 2/ SIP/Voice

Internally I want to have 2 LANS

LAN 1 PCs/Servers etc. LAN 1 only goes to Wan1 

LAN 2  VOICE - consists of a phone system and 1 phone

Management PC that should be able to see everything.

The 1 Phone needs POE so it is physically on the same switch as everything else. On the switch I've created the two VLANs.

So my plan was VLAN1 192.168.1.x and VLAN 2 192.168.5.x. 

So I started to set up a Zone to group two interfaces, but then I read about VDOMs and wondered it that was better.  

Sorry if this is a basic question. We are only a small shop.

Many thanks in advance.

Red