Dogs don't chew their food much so it could be partially digested food. If you don't feed food that is pink and oval then is he getting access to someone else's food? Cat food? Cat food is pretty rich and will make some dogs vomit.
I'd be pretty darn concerned if my dog was eating food and I didn't know what it was or where he got it. He doesn't have access to neighbours' yards? Plus of course I wouldn't want him vomitting every few days either.