Ok, here's my suggestion then. Depending upon the exact time period you are either looking for miniatures in jungle fatigues or the earlier 107 fatigues. I get the impression you don't want the former, as there are plenty of models available for those.
So the baseline figure needs an un-bloused shirt, plain trousers and some form of high boot, that can either be painted as jungle boots or the standard laced combat boot. Headgear, again depending on exact period or even individual, could be the baseball cap, Ridgeway cap or tin pot. Unstiffened Ridgeways seem quite common in contemporary photos.
Weapons choices could be quite varied but many photos show M-1 carbines where anything more than a sidearm is being carried. Early advisors might be using WW2 era webbing in conformity with both their own weapon and the equipment worn by the SVN unit they were attached to.
On that basis I would suggest that Artizan Design's Devil Brigade range is probably your best bet, in particular packs SWW604 and 605. The former has what looks like Reising SMGs but would easily convert to Thompsons by filing down the foresight. The latter are carrying M1 carbines. At least two figures in each pack are wearing shirts. All have Corcoran jump boots so suitable as combat or jungle boots.
The pants would require the cargo pockets being filed off to be wholly accurate or you could leave them on, maybe paint them up in local SVN camo. They are all in unadorned tin pots. You could swap these out for Ridgeway/ patrol caps or berets. Mongrel and Eureka both have suitable figures as head sources. Berets can be freely sourced from any number of manufacturers. You could of course just leave them in M1 helmets if depicting them in the field. The biggest visual impact will be achieved by having figures in shirts and combat or jungle boots. To get the period flavour you paint on the black and gold US Army and black and white name tags. I think that should work and probably save you quite a bit of time and effort.
Other option? Use the Commando Miniatures Indonesians with carbines as is, no conversion required. I haven't seen them in the flesh but they look the part. With helmets they would make decent early ARVN.
Okay this is very helpfull. Thanks Carlos.