North-South Cline of Continental Asians

[Mingle]

Are the Kra the ancestors of modern Thai? And Rei of the Vietnamese?

Ethnolinguistically speaking, they both share a common ancestor but aren't ancestral to one another. It's sort of similar to Hans and Tibetans.

Rei = Hlai. They're the pre-Han population of Hainan.

[Mingle]

Where would Filipinos and Indonesians plot relative to these? Who are the Dai?

Dai is the term given to people that speak Southwestern Tai languages such as those in Thailand, Laos, Northwest Vietnam, and Eastern Myanmar. The only part of China they're a majority in is Xishuangbanna, which is a prefecture in Yunnan. They were also all historically part of the Indosphere (including the ones in China).

[Mingle]

The original homeland of Vietnamese are the yellow area labeled as Ly Dynasty. Southern expansion occurred over several centuries, and Saigon was not a part of Vietnam until the 18th century.

Not all of the Ly Dynasty. The northern part was Nung, northwestern part was Tai and the southern part of the Ly Dynasty area was Muong. Kinh people were originally from the Red River Delta and then conquered the other lands later.