I would go with the 45-min connection. The transfer counter is pretty much just a desk, and you're released right back into the departures area (same level... no mazes like at HKG or NRT).

And I have to figure that if you do miss your flight that they would stick you on the next one anyway, right? The only worry I have is your luggage, and SGN isn't known for being efficient in that respect.

I wonder what Int'l transfers will be like when the new terminal opens.