The southern Chinese origin were BaiYue (Bách Việt), meaning "hundreds of Viets tribes". BaiYue was collection of tribes Viets ethnic, but were invaded and assimilated by Han Chinese from the north. Only 2 of the "hundreds" of Viets tribal are Âu Việt và Lạc Việt resisted successfully, they were Vietnamese present today.
So, we can say that the southern Chinese origin who BaiYue, and some provinces such as Quảng Đông (Guangdong), Guangxi (Quảng Tây), formerly of Vietnam's territory.
So nothing strange if they have some cultural similarities.
Also see here:
http://www.defence.pk/forums/world-affairs/172515-china-riles-vietnam-island-cruise-4.html#post2812583