By my linguistic and culinary comparisons,

I’d estimate that Thais and Viets diverged from a common source probably about three thousand years ago, coincidentally about the time that Han Chinese began emigrating southward in heavy numbers. Austronesian Proto-Malays probably diverged from that same common source about four to five thousand years ago before sailing the seas and settling islands as far away as Madagascar and Hawaii and New Zealand. Very few traces remain of that distant association, if indeed the theory is correct, but as they say, “What goes around comes around,” and Malays and Thais were destined to meet once again in the Isthmus of Kra along their current national borders. Thai curries probably come from this association. Most words in common between Thai and Malay result from the common pre-Muslim flirtation with India and Sanskrit. After their conversion to Islam, Malays even became re-established in Southeast Asia as an inter-bred race with their long-lost Cham brethren in Cambodia, also Austronesian and supposedly the original link between the Tai and Malay languages. This happened after their once-proud culture was nearly annihilated by the land-hungry Vietnamese at about the same time that Columbus was discovering America. Whether they remained on the mainland or came back is uncertain, but their aboriginal cousins are heavily intermixed with aboriginal Khmers in the central Vietnamese highlands, they also presumably a product of that original southern Chinese proto-race.