Alternative name for moon cakes?
According to the Wikipedia article on bánh pía, which cites this source, pía comes from from the Teochew dialect (i.e., Chaozhouhua 潮州話):


  The Vietnamese name comes from the Theochew word for pastry, "pia"


While I wouldn't necessarily consider this source to be authoritative, I looked up the Teochew pronunciation of 餅 here, and it is indeed pĩã so the derivation is likely to be true.

I don't know the connection between the Vietnamese and Thai "pia", but it's likely they're related too. It would seem then that the Vietnamese and Thai names both end up literally meaning "pastry-pastry".