At that point, though, the Chinese were splitting with the Soviets and making overtures to the US; this is when Nixon went to China, after all. China was not willing to start a general war over Vietnam, which is something it told the Soviets, and Hanoi, several times. China and North Vietnam had tensions which blew up when the Chinese invaded the latter after they invaded Cambodia.
China may well allow Pakistan to do it so long as it isn't anything major.
By the time Nixon went to the PRC, it was long past time for any nation that had not previously sent troops to help South Vietnam to do so; on the contrary, the tendency was all for withdrawal.
In the mid 1960's the main Sino-Soviet dispute on Vietnam was each side blaming the other for not helping the DRV and NLF
enough. "You are helping Pakistan while she participates in US aggression against Vietnam" is not a message the PRC would like to enable... And if Pakistan doesn't send troops to Vietnam by the mid-1960's, she never will. The Sino-Vietnamese communist split and the Sino-American rapprochement were well in the future then. As I already noted, not only wouldn't Pakistan send troops in the mid-1960's, it wouldn't even join in SEATO statements condemning Vietnamese Communists.
Besides which, Pakistan needed all her troops for actual and potential conflicts with India.