I think definitely the vowels. I don't know how well I can convey this, but I'll try. Even on traditional ballads or what I call "mountain" singing, which is indeed very nasal, it is generally the vowels that are sustained. The consonants provide some wonderful opportunities for ornamentation, but you really don't hear someone sustaining nnnnnnnnnnnnn. You hear something like nnaa, or you hear the consonant turn into a humming vowelled drone.

Janie