A2 buttstock, carry handle, 20" barrel, 2 piece handguard
not the real thing but it looks practically identical.
Sorry, no. No A2 parts are correct for a Vietnam era lookalike. For a lookalike, you need the "smooth" upper reciever (no brass deflector bump), no forward assist for the M16, and the three prong "wait a minute" flashsuppressor. And, of course the carry handle must be part of the upper, not a separate piece.
For the M16A1 look, you need the forward assist, either teardrop or round button is correct, and the birdcage flash suppressor.
Both use the same triangle handguards. Sights are the two position flip type rear and round post front. I do not know if any of the tritium night sight front posts were in use in Vietnam, but they were in use shortly afterwards (76 on).
The buttstock should be the solid rubber buttplate style, although some of the trapdoor ones might have been used in country by the end. Pistol grip with no finger grooves or bottom cap.
Barrel is the lightweight style with the triangle handguard cap and the early style slip ring. Some of the "heavy barrels" were used but they were not full length heavy barrels, only the exposed part was thicker. The portion underneath the handguards was the same diameter as the "regular" barrel.
Magazines should be the "strait" 20 rnd type, which was the general issue. The curved 30rnd common today was used by some troops (if the could get them) but were not general issue for infantry during Vietnam.
The Colt SP1 sporter is the period correct semi auto. Early Colt semis used all the same parts as the M16/M16A1, except that the auto sear was not installed. Later SP1s used "semi auto" internal parts, which prevented simple conversion to full auto capability.