size should not be a problem, when using them as vietnamese.
Could plastic ww2 marines be converted into montagnards or something similar?
Well you'd need to file the Killer B figures' M-14s back to Garands as the Vietnamese didn't use them.
Convert USMC to montagnards? Conceivably, bit it would be a lot of work.
Company B just released some Filipino guerillas. One or two of those look like they might work. One advantage is that Broadbent's sculpts are typically smaller than most others. The other option would again be the Commando miniatures Indonesians.
Vietnam really is a kind of half forgotten period for gaming. If someone had the smarts to do a half decent range of troops in tin hat, US stateside style fatigues and WW2 era weapons, they'd be useable for half the world in the 1960s.