It's a fairly common story; Billy Waugh recounted fighting a force of Chinese infantry around Bong Son, and that definitely wasn't the first time I heard about the Chinese having boots on the ground in South Vietnam (I think Charlie Beckwith mentioned it in Delta Force, but I could be wrong there). The Chinese were sending massive technical and advisory aid to the North, but were there frontline combat troops in the war?
There is absolutely no proof of PLA soldiers fighting in South Vietnam against American forces. China did provide soldiers to North Vietnam however. What was the most request form of armed support were AAA-crews which would help to counter the constant threat of US bombing of North Vietnam. At its peak, the 16 anti-aircraft artillery divisions (63 regiments) consisted of a total of 170 000 troops. Chen Jian writes:
The Chinese statistics claimed that these troops had fought a total of 2,154 battles, and were responsible for shooting down 1,707 American planes and damaging another 1,608.
The Chinese support was primarily restricted only to North Vietnam, which effectively prevented any hostilities between Chinese and American troops on the battlefield. The US, as we all know, never invaded North Vietnam to avoid getting themselves into a Korean War situation. The Chinese did not even allow their AAA teams to go beyond the 21st parallel. However, the Chinese were prepared to fight off a South Vietnamese invasion (obviously backed by the US) of the North if it came to that.
Other Chinese troops that served in North Vietnam were engineers to help rebuild railways and communication lines, the Chinese People's Volunteer Engineering Force. There were no Chinese pilots involved in fighting over Vietnamese territory.
For more on this, read China's Involvement in the Vietnam War, 1964-69 by Chen Jian, The China Quarterly, No. 142 (Jun., 1995), pp. 356-387.