It's easy enough to find M-14's in Vietnam in 1967, which is really the year they started converting over to M-16s, but the air cav uses the M-16 for a lot of reasons, not the least of which being that they are more compact and have full automatic capability. Air Cav was using M-16's from the word go, as an experimental unit initially and one of the first combat units in (after the Marines in Da Nang).

Funny story, as the the army was sending troops into Berlin in 1960, they hastily issued the M14 to the units going in, though they were trained on the Garand and knew the weapon well, but had to hastily zero their weapons -after- arriving in Berlin at an ad hoc range at Templehof Airport, in Berlin.

So there's a similar situation going on in a lot of Vietnam where the M-14 is there and being used, but the M-16 is being issued but the troops might not be sufficiently familiar with it...especially as basic training still involves the M-14 in this era. But the Air Cav has been using it all along.

Edit: Got a post up.

Edit 2: It should also be mentioned in this era that the M-16 is still teething; the powder and primer are not appropriate to the weapon, which was an Ordnance demand that the weapon use the ammo the Army wanted rather than ammo that worked well in the weapon. There was also insufficient training on weapons cleaning, which was exacerbated by the jungle conditions of Vietnam -- the M-16 barrel was not chrome-lined initially...whereas the M-14 was. There's a reason why veterans might still be using the M-14's that they might well have been issued when they deployed in-country at this stage of the war. Many Marines particularly kicked up a fuss about losing the M-14 around this time.

Another issue is the government-issued magazines. Failure to feed and extract properly is the big problem with the M16, and part of that has to do with the quality of magazines. Most infantry of the era loaded 18 rounds or less in the 20-round magazines.

This all came out in a Congressional investigation in 1967 where they essentially asked, "Why does this wonder-weapon have such a lousy reputation in the field?"

Also, the M60, by the same standard, has its flaws as well.

Further edit: However, at this time, thanks to the work of MACV-SOG in salting enemy ammunition supplies along the Ho Chi Minh with defective ammo, enemy AK's have a chance of blowing the bolt right back into the face of the trooper. It's probably also a problem with SKS'es and RPD's which use the same round as well.