I know many of us here thought Muong were the ancient Viet, but the following series of articles I've just found suggest that they weren't. Well they were one of the ancient Viet/Yue tribes in term Bách Việt or Bai Yue but they weren't the same tribe as us Kinh people.
http://www.khoahoc.net/baivo/nguyennguyen/...haigiongmau.htm
http://www.khoahoc.net/baivo/nguyennguyen/...ngchungthai.htm
http://www.khoahoc.net/baivo/nguyennguyen/...uachungthai.htm
The author of the above articles say that they were the ancient Tai, who arrived first at North Vietnam (before us Kinh). When the Han conquered North Vietnam they fled to the mountains and there they got mixed with the Melanesian.
In summary he based his argument on the followings
1. Muong language has many similarities to Tai languages (read the articles to see details)
2. Muong culture is very similar to Tai culture
3. Muong people worship Ngu Ko (Âu Cơ) but not Luong Wang (Lạc Long Quân), they regard her as the founder of their people. Âu Cơ was an ethnic Tai (Âu people).
4. Muong people also worship Thục Phán. And we know Thục Phán was an Âu Việt (Tai ethnic). Tày people said Thục Phán was their "thủ lĩnh".
I remember that one of the words Muong people use to call themselves is Au Tá. Au ~ Âu, Tá ~ Táy (Thái) ?
And the article from this link http://www.vietnhim.com/dongnhim/showthread.php?t=19813 also says Muong people are of Tai ethnic.
So, any comment? It's quite confusing.