Everyone learnt from the Korean War and wished to avoid a repeat of a bloody direct Chinese-American fighting. At the onset of the escalated American involvement in 1965, Beijing made it clear where the Chinese line in the sand is:
[I]f the Americans went beyond the bombing of the North and used ground forces to invade North Vietnam, China would have to send military fores. Second, China would give clear warning to the Americans.
- Harper, John Lamberton. The Cold War. Oxford University Press, 2011.
The message was clearly received in Washington D.C., where American leadership were no more eager to fight the Chinese against. The United States thus undertook to avoid crossing that line.
The near-certainty of a second war with China deterred the Americans from taking the ground war beyond the 17th parallel.
- Harper, John Lamberton. The Cold War. Oxford University Press, 2011.
Now, there is definitely some controversy over whether the Chinese were bluffing, and whether America should have, or could have, invaded North Vietnam to secure a quick victory.
The decision of the Johnson administration not to invade North Vietnam, then, was based on a correct evaluation of the threat of direct Chinese military intervention, as well as on a well-informed gear of provoking a Soviet-American confrontation.
- Lind, Michael. Vietnam, the Necessary War: A Reinterpretation of America's Most Disastrous Military Conflict. Simon and Schuster, 2013.
In contrast, revisionists dismiss the Chinese threat and lambast the American leadership for not taking the ground war to Hanoi.
General Dave Dave Richard Palmer writes: The Johnson administration had already barricaded the one sure route to victory - to take the strategic offensive against the source of the war. Memories of Mao Tse-tun's reaction when North Korea was overrun by United Nation troops in 1950 haunted the White House ... Summers contends that the US was "bluffed" by China.
- Hess, Gary R. Vietnam: Explaining America's Lost War. John Wiley & Sons, 2015.
Regardless of hypothetical scenarios, ultimately the United States never attempted a ground invasion of North Vietnam. Thus, bluff or genuine promise, China was never confronted with the choice of whether to follow through with their threat of intervening in Vietnam.
Whether Peking's threats were genuine or not, American presidents prudently refused to risk such high odds. North Vietnam remained inviolable to ground attack.
- McNeill, Ian, and Ashley Ekins. On the Offensive: The Australian Army in the Vietnam War, January 1967-June 1968. Allen & Unwin, 2003.
That is not to say China didn't assist the North Vietnamese. Even though China and America did not end up engaged in open warfare, large numbers of Chinese soldiers and military materials flowed into Vietnam. In fact, at the peak of China's involvement, over 170,000 Chinese military personnel were in North Vietnam, not to mention much needed equipment and supplies.
Between August 1965 and March 1969, sixteen Chinese anti-aircraft divisions, amounting to 150 000 personnel, operated in North Vietnam. The total strength of Chinese troops in the DRV reached a peak of 170 000 in 1967.
- McNeill, Ian, and Ashley Ekins. On the Offensive: The Australian Army in the Vietnam War, January 1967-June 1968. Allen & Unwin, 2003.
