Yeah, I don't think many used the AK47 for multiple reasons, primarily as August said, in a low visibility scenario, you shoot in the direction of the sound of enemy fire, which if you're firing an enemy weapon...
You're gonna have a bad time.
Also, there was a campaign to sabotage Viet Cong equipment, including ammunition, along the Ho Chi Minh trail, so there was a small percentage, but still a chance, that the AK47 you picked up might choose to spontaneously combust in your hands whilst firing.
I would imagine that some did though, but I don't think it was on that large a scale.