Full disclosure, I've never seen these before now, so I put my internet search skills to the test. I think the Vietnamese name is Quả chùm chày, roughly "Fruit of the Pestle." I have seen a couple of websites where they were referred to as "pomegranates" but that might be due to a similarity in the flavor or the flesh/seed ratio. Looks like it might grow wild in Vietnam, don't know if it has been cultivated.