I doubt anything I'm about to mention hasn't been mentioned, and I'm not really an expert on the Vietnam War, but here goes:
As it stands I think the only real weakness of SOGPF's arsenal is in ground vehicles. Outside of tanks and trucks, there isn't really much symmetry between the NVA and US in terms of what broad ground vehicle categories they get. So here goes:
For light vehicles: M151 Jeep for US, GAZ-69 for NVA. This is a fairly blatant hole afaik as the NVA don't really have any light unarmoured patrol vehicles. The GAZ-69 is a bit bigger than an M151 but that's fine, the M151 is tiny.
I can't actually think of a truly appropriate BTR-40 analogue as a light armoured wheeled APC other than maybe an unarmed variant of the armoured M54 gun truck or some other, lighter truck chassis like the Jeep M715 with an armoured troop carrier variant, but neither of those seem particularly historical in terms of ever having existed at the time.
For APCs: M113 for the US, BTR-50 for the NVA. The M113 is an iconic vehicle with plenty of variants (like the ACAV turreted version) and is definitely the recommendation I hope most makes it in as it would fill a huge hole in the US arsenal. A BTR-50 would also be nice as a larger, less soft APC for the NVA to sometimes use, but is probably less integral to the setting than the M113.
I'm not really sure where to go up from there. Full MBTs like T-55 vs. M48 would be nice I guess but the current M41 vs. Type 63 pairing is adequate for most scenarios. NVA jets could be nice from a historical perspective as they participated a great deal in the air war, but I don't play air stuff and don't know how much dogfighting would really factor into most Arma gameplay scenarios. Heavy helicopters would definitely be nice - CH47 and Mi-8 - but also not quite as profound a change as the ground vehicles above. I have literally zero problems with the current variety of guns unless you want to add the Australians as a faction.
In terms of off-topic gameplay gripes I've found that the weights of some ammo loads are off - AK ammo is tied with Stoner ammo for lightest per-bullet of the assault rifles (weighing as much as 28-round M16 mags when of course IRL AK mags are heavy as hell), and the M60's ammo is significantly lighter than even the RPD's (like I'm talking 500 rounds of M60 ammo for every 400 rounds of RPD). Also the M38 and M1891 Mosins could do with stripper clip reloads since they don't have a scope to get in the way 😁
tl;dr: add, bare minimum, the M113 and a light utility vehicle for the Vietnamese like the GAZ-69.
Edit: Also add the Ithaca 37. Historically the M1897 being in the DLC makes sense, but having it be the main US shotgun seems odd to me when from what I know the Ithaca 37 was in much more widespread use, what with it not being leftover stock from WW1.