Different-tasting coffee in Vietnam/Cambodia When traveling in Vietnam and Cambodia, I noticed that the coffee had a very different taste, compared to anything I've had in the U.S.. I'm talking about black coffee, not Vietnamese coffee with condensed milk. It's hard to describe the difference; it seemed a little bitter or sour.

What accounts for this different taste? Is it arabica vs robusta? Or just a local difference in the beans?
"Robusta coffee accounts for 97 per cent of Vietnam's total output" - Wikipedia

It's very likely you were drinking local pure robusta coffee.  Among other things, the increased caffeine content makes it more bitter.  But, robusta flavor is also generally considered inferior to arabica.