I haven't been around WoW for a bit ,so I might've forgotten something.
From what I remember to get vertex lighting you have to enable the flag in the MOGP chunk (optionally the indoor flag too if it's indoor) if you leave it outdoors the light will just blend with it like it normally would.
You then need to add a MOCV chunk yourself, (The MOGP and MOCV are in the group file, I think I edited MOGI in root file when testing indoor/outdoor ,but you couldn't see the inside unless you moved into it because of  no portals) After that I used "4 bytes per vertex"(quoted from wowdev) and trial and errored it.  That's how I got light in my .wmos .-.
Old examples attatched -edited