We are planning a trip to Vietnam, Cambodia and Laos, and have contacted a couple recommended tour companies to develop itineraries. These are Vietnam based agencies, and to my surprise, they only require a relatively small deposit upfront - and the balance is paid on arrival. This strikes me as odd, though in every other way they have checked out as reputable. Can anyone tell me if this is common for Asia? I'm open to pleased referrals from past experience as well.

Somewhat common and actually well to your benefit.

It is common is some parts of Asia (Asia is a big place), like VN.

It's not unusual in Vietnam. So, if you find them reputable, just continue to book your trip.

I went to the exact same countries using Aurora/Vietnam Stay to plan all. They are based in Vietnam. We had a large group and ultimately we only paid 50% up front and the rest in country. And we didn't even do it right away! We didn't pay until the morning of our 3rd day when they found it most convenient for our itinerary!

Enjoy your vacation.