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.
"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.
I'm pretty sure the coffee you were drinking was a coffee and chicory mix.
I had alot of it in Vietnam when I was there.
In Vietnam about six types of coffee blends are available, it is a coffee producing country.
One of them is an Arabica coffee, usually served in restaurants and sold at the open markets on the street. If you ask for Arabica, you will get this variant. It does take some communication skills using your hands and feet to make it clear to market salesmen. In Vietnam only the young and the elite speak English. You should be able to buy a drip filter:
The other five are local products. Some of them are said to taste like chicory. Anyway they have a bad taste if you're not used to them. But they are really not chicory, they are blends from indigenous coffees like Arabica SE, Chari (a type of Liberica), Catimor. Also Arabica and Robusta are produced in Vietnam.