Not considering things like vocabulary or syntax (if that is possible), what phonological differences make the most telling distinction between two dialects, is it the vowels or the consonants?
Informally, among English dialects, it seems that between any two things called dialects the major differences are the vowels (and the consonants seem to stay the same).
But I've heard (unsupported of course as is my personal judgement) that in other dialect families, it is a change in consonant (and not the vowel that marks the dialect difference) (the classic example being the 'shibboleth', an s/sh difference.
(of course this may be a false dichotomy, and there are other phonetic differences that are not about vowel/consonants, like say prosody or accent).
This question is somewhat motivated by my other question about lexical sets; that is an analysis technique for managing multiple varieties purely by vowels. Maybe that method is useful in general (in other sets of language varieties), or maybe it is just helpful for English, or maybe it is just one tool, and other tools are more or less successful (and I'd like to know to what degree).