In the first place, any reasonably detailed discussion of the visits of Nixon and Kissinger to China will note that one of their goals was to harm the DRV's struggle. As one book notes, both men knew that "by opening to China, they would be able to make North Vietnam feel more isolated and vulnerable." (Isaacson, Kissinger: A Biography, p. 334.) And "the North Vietnamese had drawn the inescapable conclusion that China valued its relationship with the United States more than its revolutionary unity with the DRV." (Qiang Zhai, China and the Vietnam Wars, p. 197.)
Kissinger in his memoir Years of Upheaval noted that China "profoundly distrusted" the DRV and that if the Paris Peace Accords failed "Hanoi would achieve hegemony in Indochina without a fight, discredit the United States internationally as a paper tiger, and create on China's southern border a powerful Vietnamese state... in Indochina, American and Chinese interests were nearly parallel... Zhou Enlai... had always urged a ceasefire much like what we had achieved, the implication of which inevitably would permit the South Vietnamese government to survive. Unlike many of our domestic opponents, he never pressed us to overthrow Thieu and to install Hanoi's puppet regime."
In Mao: The Unknown Story (yes, I know it's a dubious source, but this bit is confirmed by other sources) the authors relate the following on page 585: "When Chou went to Hanoi immediately after Kissinger's first visit, to explain Peking's move, he got an earful from North Vietnam's leader. 'Vietnam is our country;' Le Duan protested; 'you have no right to discuss the question of Vietnam with the United States.' After Nixon's visit, Chou returned to Hanoi, and got an even worse reception."