Originally Posted by
ok986
The thing is that VN has a 9 am flight to HUI and then a 3pm one, so I bet on the fact that we may be fast

Is there a direct drop-off in the baggage collection area, like at the big US airports? How much time does immigration usually take?
Would not a bag drop upon exiting customs slow you down? That adds one more line (since you still need to hit the checkin gate at the domestic terminal anyway).
My recollection from about 2 years ago was that it took me well over an hour gate-gate w/o bags.
The good news is that they will not be a stickler for any sort of "be there by T-XX" stuff. If you get to the gate and the plane has a seat they will get you on.
The bad news is that you can not walk fast through lines. And you are looking at several.
If you try this, be prepared for the 6 hour layover if it fails.