Vanilla override does not seem to work with drops.
I'm using the latest MythicMobs and MMOItems Dev Builds as of last night. Essentially, if you go into the VanillaMobs.yml and try to implement an MMOItems drop, the mob will never drop it under any circumstances, regardless of if the mob is spawned naturally or by command.
I've tried a simple override such as this:
PIG: Drops:
- mmoitems{type=MATERIAL;id=PIG_CARD} 1 1
- stick 1 1
And, at least in my tests, I can't even get the stick to drop.
EDIT: Sorry, I'm pretty new with the whole GitHub thing and it messed up the formatting pretty bad. Err, best I can do is say that rest assured, it's implemented properly lol