Just did this but from CXR (Nha Trang) to SGN.
we flew on to Melbourne with VN.
​​​​​They were able to check the luggage through to final destination so they won't be a problem for you.
Personally I wouldn't book anything under 2 hours mainly for a buffer for delay.
if you transit SGN when you leave domestic turn left and head for the internation terminal. When you get there go past highlands coffee and turn left at Burger King.
There is an entrance there to go up to the departures level, go up 2 escalators, then you will need to exit outside to then re enter the terminal.
Apparently business passengers with VN can use the diplomatic/crew immigration lines which can save a bit of time, we didn't know this but the immigration officer mentioned it.
After immigration go to the far right for security that is the fast track lane.
As for Hanoi I haven't done a transit there but I do know the terminal there is further from domestic but much newer.
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