Actually The Six-Point Directive was issued by CCP Central Military Committee, not by Mao himself in Jan 11th, 1965, because it does not want engage in a war with US on the premise that US will not attack North Vietnam. However after US aircraft flew into China's airspace over Hainan Island between April 8 to April 9th the same year, Mao lifted the order not to attack US aircraft in April 9th by the request of Head of PLA General Staff Headquarters Yang Chengwu. It is quite possible that further US escalation of the war in Vietnam in the intervening monthes also caused Mao to abandoned the restriction against engaging US aircraft.
Then Starting in 1965, China sent anti-aircraft units and engineering battalions to North Vietnam to repair the damage caused by American bombing, rebuild roads and railroads, and to perform other engineering works. This freed North Vietnamese army units for combat in the South. Between 1965 and 1970, over 320,000 Chinese soldiers served in North Vietnam. The peak was in 1967, when 170,000 were stationed there. However China's involvement was not as commited as it did in Korean War. It also withdrew its troops early starting in 1968 and completed its withdrawal in 1970 because of the upcoming conflict with Soviet Union.