The Kalashnikov was fairly rare early in the war. I believe that it began to be encountered in significant numbers around 1965, especially with NVA forces, but did not become entirely ubiquitous, especially among the Viet Cong, until somewhat later. Various other small arms were used, especially the SKS semi-automatic, the Mosin-Nagant's various iterations, French-captured arms, and Soviet and Chinese submachine guns.

As to the effect it had on American forces, see the quote below, from David Hackworth, a career army officer who had spent two years fighting in Korea, and would spend three years fighting in Vietnam.

At one stage of the work, one of the dozers uncovered a slightly moldy VC body, complete with an AK-47. I jumped down in the hole and yanked the AK out of the mud.

"Watch this," I said, "I'll show you how a real infantry weapon works."

I pulled the bolt back and fired thirty rounds - the AK performed as though it had been cleaned that day rather than buried in a marsh for a year.