The M113 is good. It is all metal, top and bottom half are separate as well as tracks. Does need a little gap filling. It has options to run it as an ACAV as well included with 2 M60 gunners in the back. Eureka also do a version.
As for the ARVN that is the only picture I have seen. I know when I was looking for NAM figures I couldn't find any pics of the ARVN. I do know from others that they come with M16s only so would be suited to later in the war. I know that Jungle Rot did ARVN with M14s but I haven't seen any in the flesh.