Bom dia. As far as I know, Khmer nouns don't have plural forms. You either depend on the context or use special words to emphasize that you don't mean just one person.
So, if you want to say "a lot / a group of Vietnamese", for example, you may say ពួកយួន or ពួកវៀតណាម. I'm not completely sure, but you can google both phrases. And there are very few people at this forum interested in Khmer.