Based on what I know of Chinese history, Southern China was settled by the Han much later than the North and the people in the South were considered barbarian referred to as the various types of Yue (known as the 100 Yue) in later times. Eventually the people region that became Guangdong and North Vietnam were referred to as (Nan Yue, or South Viet). Most of these people were likely Austroasiatic speakers in origin (like present day Vietnamese and Cambodians). Since Northern Vietnam (Annam) was part of China on and off for over 1,000 years; and the south, by the end of Chinese colonization was controlled by Champa, a Malay people (Austronesian).
As far as I know there was a massive influx of Han Chinese into the region during the Song Dynasty due to Barbarian pressure in the north. I know assimilation was fairly complete by the Tang Dynasty as Cantonese speakers often call themselves Tong (Tang in Mandarin) People and talk of giving their children Tong names. They also still refer to their province and themselves as Yue to this day. Im guessing by the Late Tang, the Sinization of the area was complete, but for Annam. Vietnam became independent from China after the disintegration of the Tang, since the Viet or Yue people lived in what is now Guangdong as well, Im guessing by that time the people in Guangdong were mostly Sinized, and considered themselves Han Chinese, but most of the people further South did not.
Also, South, in China is the area from Shanghai down to the border of the Southeast Asian nations of Laos and Vietnam.
Other nonHan ethnicities lived in the South, such as the Lao/Thai (Tai-Kadai language group) folks also came from Central China and were pushed South by the Han, they still have relatives in modern China like the Zhuang and Dong peoples.
To wrap it up, it is not shocking that Han men (like many men before them all over the world) would move to an area and take it over, while enslaving, killing, or running off the native men using their superior technology and social organization. Then they would marry, rape, or concubine the local women. Men, historically, are not picky about who they have sexual relations with. In a desperate spot any woman (even a barbarian) will do.
This new study provides more detail to earlier studies whose results where along the same lines.
European Journal of Human Genetics advance online publication 23 January 2008; doi: 10.1038/sj.ejhg.5201998
A spatial analysis of genetic structure of human populations in China reveals distinct difference between maternal and paternal lineages
Fuzhong Xue et al.
Analyses of archeological, anatomical, linguistic, and genetic data suggested consistently the presence of a significant boundary between the populations of north and south in China. However, the exact location and the strength of this boundary have remained controversial. In this study, we systematically explored the spatial genetic structure and the boundary of northsouth division of human populations using mtDNA data in 91 populations and Y-chromosome data in 143 populations. Our results highlight a distinct difference between spatial genetic structures of maternal and paternal lineages.
A substantial genetic differentiation between northern and southern populations is the characteristic of maternal structure, with a significant uninterrupted genetic boundary extending approximately along the Huai River and Qin Mountains north to Yangtze River. On the paternal side, however, no obvious genetic differentiation between northern and southern populations is revealed.
Lol! 
Those hippos remind Marshmallow of herself, because that's what became of her after eating way too many sweets (marshmallows). 
Beware of the ugliest duck face ever.
i quoted u seperetly not wid dat post to levina but it came out wid it
