1911greg,
In the early part of the War, Type 56 rifles (Chinese version of the AK-47) weren't available in the South. Many NVA units, in fact, were equipped with SKS rifles at the beginning of American involvement.
Logistics was the problem for their fielding. The Ho Chi Minh Trail wasn't established as well as it was during the later war years. In fact, until around 1965 or 66, you'd still find MAT-49's and even stuff captured from US forces in Korea in the South with VC units and infrastructure.
My timeline may slightly be off, but by and large, the SKS rifle served the VC well from 1964 until 1970. Main Force Units received AK's around 1966 or 67. Then the Soviets started shipping stuff into the country. This also tied in with the falling out between Vietnam and China.
Shelby Stanton has written several great books about arms and the Vietnam War. One of my favorites is "Vietnam: Order of Battle." It shows all units (down to battalion level) that served in Vietnam, and when.