A couple of years ago SWMBO and I did a Cambodia/Vietnam trip. On a single visa.
Flight was in to Vietnam, and on to Cambodia with a 2 hour layover. But......and it was a huge but.............The bags could not be inter-lined.
I rang the travel agent about this, and spoke to some one who had had their ears removed, and was in broadcast mode. I have travelled a lot in Asia, I told them, and I know the rules.
I was, of course, told that I was wrong.
I should have put it all in writing..................We arrived in Vietnam, had to go and get our bags, and used up our visa, just for getting the bags. There were no options for transit-style visas.
On the tour, for the next entrance in to Vietnam, the Tour Guide had to fix it all at the border. I hope it cost them an arm and a leg.