Now, this is tough.
I have come across many dialects and languages but few match to the complexity of the Vietnamese language. Living and working as an expat in Vietnam demands knowledge of the local language used on a day to day basis. Although, a past French colony before, Vietnam has not retained much of the language in its current use. The national language has always remained predominent and even the 50 ethnic dialects or so still stay behind.
However, I should share my candid views about this queer lingo. Fist of all, just as consonants rule in Welsh, vowels in Vietnamese take a myriad of accents borrowed from the French to give a challenging gamut of sounds to your poor epiglotis. Here is a sample, can you hear the 12 accents in the sounds ?