When humans are infants, they learn what phonemes are in their native language, and they learn to distinguish between them. If someone isn't exposed to different phonemes being distinguished at a young age, they may be unable to distinguish them at a later age. For a native speaker of English, "l" and "r" are clearly different, but apparently for people who aren't exposed to this distinction at a young age, they sound very similar. Asians in general seem to have trouble with this distinction, leading to the term "Engrish" being used to refer to English as spoken by Asian people. I'm sure there are phonemes in Vietnamese that native English speakers would have trouble distinguishing.