For your situation, the child will born on VN soil and the mother still VN citizens, so the child will have VN citizen and can apply for VN passport. After that, CBRA need to submit to US Embassy in Saigon and the child will have US passport.
The baby will fly to US with those two passports (VN passport to fly out, and US passport to get in US). However, nature born USC is what I'm always prefer any parents do. If holding dual citizenship, it will prevent your child hold some high level federal jobs (you child may become US president in the furure, so..)
In the end, it's your personal choice.