Thanks to all the responses from people who listened to the short audio I posted the other day of a man "speaking strange Vietnamese".  I got all kinds of answers from "it's Vietnamese" to "it sounds like Thai mixed with Vietnamese" to "it sounds like a Thai or Cambodian man speaking Vietnamese".  Some people actually understood the content quite clearly, which talks about Jesus, while others couldn't understand it at all.

In fact, the segment was neither a man speaking Vietnamese nor a foreigner speaking Vietnamese with an accent.  This man was speaking his native language, which is Mường, a language closely related to Vietnamese, and perhaps closer to the origins of the Vietnamese language.  The Mường people are basically ethnically Vietnamese but live in the mountainous areas of North-Central provinces such as Hoa Binh and Thanh Hoa.

Although considered a separate language from Vietnamese, many Vietnamese speakers will find they can understand much of the Mường language without studying it.

For example:

Mường: Ăn cơm thơi, tỏl rồi.
Vietnamese : Ăn cơm thôi, đói rồi.

The language becomes easier to understand if you consider the following sound correspondences.

s - kh: Vietnamese 's' should be pronounced distinctly from 'x', unlike the Hanoi accent.  But in Mường it's pronounced like 'kh' so Vietnamese "sông" becomes Mường "không".

b - p: Vietnamese 'p' and 'ph' used to be more similar, with a puff of air added in pronouncing 'ph'.  There are no native Vietnamese words beginning with 'p'.  Vietnamese 'ba' is Mường 'pa'.

d - t: Vietamese 'đi' is Mường 'ti'.  There is a basic pattern of Vietnamese initial stops becoming voiced versus unvoiced initials in Mường.

t - th: Vietnamese "tay" is Mường "thay".

g - k: Vietnamese 'gà' is Mường 'cà'.

tr - tl/bl: Vietnamese doesn't have the consonant cluster 'tl' but it did have a few remaining words beginning with 'bl' until recent history.  Vietnamese "trời" is Mường "blời", which was also a Vietnamese word at the time Europeans arrived.  Vietnamese "trong" is Mường "tlong", "trăng" is "tlăng", etc.  This will seem more strange to Hanoians, who pronounce 'tr' as 'ch'.

The tones are also different.  Mường has five tones whereas Northern Vietnamese has six.  Like most Vietnamese dialects, Mường can mix up the hỏi tone with ngã.  Furthermore, Mường has no nặng tone.  Vietnamese words with nặng tone would have dấu sắc in Mường.  (In the Nghệ Tĩnh dialect of Vietnamese, the nặng and ngã tones are also mixed up.) But the tones are even pronounced different from Vietnamese.  According to Wikipedia, Vietnamese rising tone, thanh sắc, and falling tone, thanh huyền, are reversed in tiếng Mường!

Regarding vocabulary, perhaps the most important thing to remember is that Mường didn't undergo such heavy Chinese influence, where Chinese words make up about half of the words in a Vietnamese dictionary.  So don't expect a Mường speaker to use much Hán-Việt, except for certain words borrowed from Vietnamese like "quốc" or "chính".

So now try listening to that audio again and see how much of this Mường "foreign" language you can understand!  Or head up to Hoa Binh or Thanh Hoa to study the easiest foreign language you will ever learn!