Short answers, pulled from the story.
The Cambodian, Vietnamese War began on the 21st of December 1978 when a Vietnamese offensive crossed the border. The war officially concluded on the 26th of September 1989 after Vietnam withdrew its remaining soldiers from Kampuchea.
General Le Duc Anh authored the K5 Plan and led the Vietnamese campaign in Kampuchea. He commanded eight divisions estimated at 60,000 soldiers to launch the retaliatory strike against Kampuchea.
The UN Credentials Committee decided to recognize Democratic Kampuchea by a vote of six to three. This decision allowed representatives of Democratic Kampuchea to be seated in the General Assembly with strong support from China despite objections from the Soviet Union.
Of the 1.26 million regular soldiers under arms, 180,000 were stationed in Cambodia in 1984. By 1985, the majority of ten Vietnamese divisions were assigned to operations on the frontiers while the remainder stayed in major provinces.
On the 23rd of October 1991, the Vietnamese Government signed the Paris Peace Agreement which aimed to restore peace in Kampuchea. This treaty opened the process of reconciliation between Cambodia's factions leading to UN-led transition and restoration of multi-party rule.