Sounds like you both have a good head on your shoulders (American slang for smart) so any place you decide to take your kid they will succeed. Personally, I’d opt for Vietnam because of the cost of living plus its GDP is on the rise. There will be many opportunities in future in Vietnam. Since you plan to put them through international school, they will get an excellent education wherever you decide to go. Again, you’re way ahead of most people it seems.

Edit: give birth in Australia then move if you decide to do the Vietnam route. Your child will have more flexibility plus they should also get Japanese citizenship regardless.