We'll be getting visas ahead of time. I know that getting the approval letter and picking the visa up at the airport is an option but I just want to have it all done ahead of time and am willing to pay more for that peace of mind.

My question is: we're flying into Vietnam then at the end of our trip we fly to Cambodia. After our time there we fly back to Hanoi to get our connecting flight home that same day. We won't be leaving the airport in Hanoi.

I'm not sure if that qualifies for single-entry or not. I wrote to the embassy and was told:

IT'S NOT COUNT AS 1 ENTRY IF YOU DO NOT LEAVE THE AIRPORT/ GO THROUGH IMMIGRATION

I would assume that since the Siem Reap to Hanoi flight is international we would not go through immigration to get to the other gate and so single-entry would be okay. But is there a possibility the airport is configured in such a way that we will have to go out and back in -- or a possiblity our flight could get cancelled and we'd need to leave the airport that night and go back the next day - I guess there is always that possibility.

So with those scenarios would you go ahead and get multi-entry visas just to be safe? As I said, I'm (overly?) big on peace of mind and would rather pay for that than wait & get jammed up. I guess I've answered my own question.

it seems to be a one entry to me as you will not be passing through immigration again....

You really need to find out how the airport is configured.

There are airports where you do have to pass through immigration in a situation as you describe. I wish I could remember the situation in Hanoi but I don't.

Yes, I can envision having to do that in smaller airports. I think just to be on the safe side I'll get the multi and not have to worry. I don't want to think about the headache/nightmare/delay if for some reason it was needed and we didn't have it.

You dont need to get Multi entry visa.
There is a desk for Transit Passenger when you get off the plane in Noi Bai airport.

It's a small airport so it will be boring if your next flight is delayed.

OK, so I've been assured by the Vietnamese travel agent, the embassy and some of you guys that single-entry is all that's needed. I guess I should believe it!