Problem is, when the latin alphabet was applied to the oral Vietnamese language centuries ago, (by French priests, if I'm not mistaken) they used the "r" because it sounded closer to the spoken sound they heard than any other consonant available to them. The hint of a "z" you hear is actually an accurate representation! You will also hear a "z" sound in one dialect for the Viet letter "d" (as in "da phai") for one dialect and a "y" sound in another dialect. Same words, different pronunciations, depending on where in Viet Nam one comes from. At least, this is what I was told 50 years ago when I studied the language.