Even ignoring the issue of tones though, spoken Vietnamese is pretty rough for ears used to English. Vowels are extremely hard to distinguish, and the fact that almost all words are one or two syllables means each phoneme matters a lot, there's less room for error in pronunciation to still remain comprehensible.
Indeed, as the article briefly touches on, the similarity between the pronunciation of different words is something evident to the Vietnamese themselves, and word-play using this is the basis of a lot of poetry and humor.
