Excerpts from the Black Paper
As Kampuchea's delegation always kept silence about the Paris negotiations, after two or three meetings the Vietnamese asked for its opinion. The Communist Party of Kampuchea's delegation put back the question in return: "Whom to negotiate with? Would we have to negotiate with the Lon Nol clique?" But the latter was already dying. . . . "Would we have to negotiate with the US?" Kampuchea's revolution had not to negotiate with the aggressors of Kampuchea. . . . Besides, the Communist Party of Kampuchea had nobody to carry out negotiations.
The Vietnamese replied: In our opinion Kampuchea's comrades must negotiate. If Kampuchea's comrades have no cadred to carry out negotiations with the US, we can do it in their place.
The Vietnamese impudence is boundless!
In October 1972, Vietnamese pressures became more imperious.
In fact, the US and Vietnamese had already put the broad outline of the draft of Paris Agreements into shape. Pham Hung and Hay So asked to meet once again with the Central Committee of the Communist Party of Kampuchea. The talks lasted four days instead of the initially scheduled two days. During the talks the Vietnamese have shown their rare insolence and were in a towering rage. Both sides expressed once again their points of views and each side kept abaiding by its position. The Communist Party of Kampuchea yielded nothing under the vietnamese pressures. 
. . .
At the moment when he was about to go home, Pham Hung said he had been entrusted by the Vietnamese party to inform the Communist Party of Kampuchea that on the day of their meeting to finish off the text of the Agreements, Kissinger asked Le Duc Tho to inform the Kampuchea's side that if Kampuchea did not cease fire, the US strategic and tactical planes would destroy Kampuchea within 72 hours. This was an open threat uttered to the Communist Party of Kampuchea. . . .
When Comrade Secretary Pol Pot came back home, he received a letter from the Vietnamese party to South Vietnam. . . [which] only confirmed Kissinger's threats underlining that if Kampuchea did not cease fire, he would totally destroy Kampuchea's revolution within 72 hours. The Vietnamese asked the Communist Party of Kampuchea to more carefully consider the problem. Did Kissinger really talk like that? Probably. But anyway, the Vietnamese were involved in this affair. . . .
As it has been mentioned above, the Communist Party of Kampuchea did not know with whom to negotiate, for Lon Nol was already dying. As for the US, they were the aggressors. They had to stop the aggression? . . . Besides, a cease-fire would spread confusion in the determination of the people and the Revolutionary Army of Kampuchea in waging their struggle.
On the other hand, at the end of 1972, the political situation of the whole of Southeast Asia showed that it was in Kampuchea where the revolutionary situation was best. South vietnam was under the Thieu clique's control. It was the same in Laos which, except some regions, was controlled by the administration of the Vientaine. As for Kampuchea', the Kampuchea's revolution, on the whole, grasped the situation well in hands and controlled the country. If the map was coloured, black colour would be in every place, except in Kampuchea were red colour would dominate. The objective of the US imperialists was to take this red place and turn it into black colour. . . . If Kampuchea's revolution failed, Vietnam's revolution would also fail. It would be the same for the other revolutions in South East Asian countries. . . .
So, when the Vietnamese informed the US that they had failed in forcing Kampuchea to negotiate and cease-fire, the US were very mad and decided to send their B-52 to bomb Hanoi in December 1972, until the Vietnamese implored them to stop bombing and resume negotiations. 
. . .
Kampuchea has been totally and definitely liberated on April 17, 1975. South Vietnam has been liberated on April 30, 1975. The Vietnamese had to leave Kampuchea and go back home. The Communist Party of Kampuchea requested the Vietnamese to withdraw before the end of May 1975, and at the lastest, at the end of June, 1975. But in fact, only one party of Vietnamese withdraw from Kampuchea. . . .
It was in Ratanakiri province where they were most numerous to remain in Kampuchea's territory. There were more than 1,000 scattered here and there in many places by groups of ten to one hundred. . . . Kampuchea's regional forces requested them to withdraw. The Vietnamese replied that the territories located North of Andaung Meas and Voeunsay were Vietnamese territories. . . . In Mondulkiri province, the Vietnamese troops also refused to withdraw. They finally withdrew only under the threat of the regional Secretary to drive them out by force.
At Snuol (Kratie province) the Vietnamese accepted to withdraw for they were aware of the measures taken in Ratanakiri and Mondulkiri provinces. 
1. Black Paper: Facts & Evidences of the Acts of Aggression and Annexation of Vietnam Against Kampuchea, pp. 72-74. Phnom Penh: Department of Press and Information of the Ministry of Foreign Affairs of Democratic Kampuchea, September 1978 (English as in original).
2. Ibid., pp. 74-46.
3. Ibid., pp. 78.