There is undoubtedly some stylistic influence, particularly in some Vietnamese and Cambodian pieces that have a more uniform breadth of the blade (i.e., no "belly") and a small round guard, and there are some blades of a definite SEA style that appear to have been made using a classic Japanese differential hardening technique to create a distinct billowy hamon (see below). However, the basic blade form in all likelihood predates any significant Japanese influence, and what you see is just that, influence, not derivation. Chinese influence is very pronounced in Vietnamese "Chocin sabres."
There was an interesting historical event in Thailand, which, I think, shows the essential independence of the dha/daab as a style from the Japanese katana - Japanese-style swords had become so popular (I am without my notes at the moment, so the exact timeframe escapes me - I think it was 17th C) that the king issued an edict limiting their wearing unless they showed at least some Thai decorative features. He apparently felt that the traditional Thai daab was being threatened by the popularity of the katana.
