THE Yugoslav civil war has entered a decisive phase with the end of United Nations mediator Cyrus Vance's latest mission. The main protagonists face the choice of embracing his plan for peacekeeping forces or facing unabated violence."I think they all understand that the Vance mission is the last chance they have, that a failure of the Vance mission could result in a major escalation of the war" says a Western diplomat here. Mr. Vance has for now ruled out the deployment of UN peacekeepers because fighting between Serbs and Croats has continued in disregard of a Nov. 23 cease-fire accord; the accord's implementation is the main condition for UN Security Council approval of the plan. But the plan remains the only option for halting the savage five-month-old conflict, and despite the failure of Vance's exhaustive attempts to secure the cease-fire, the UN is expected to pursue its peace efforts. The fate of the cease-fire now rests firmly with separatist Croatian President Franjo Tudjman, Communist President Slobodan Milosevic of the rival Serbian Republic, and Yugoslav Defense Minister Veljko Kadijevic, the most senior general of the Serb-dominated federal Army. Western diplomats and other analysts said all three have indicated they might be willing to cooperate to solidify the cease-fire, because of growing public weariness of the war and worsening economic conditions wrought by international sanctions. One indication is Vance's surprise disclosure on Sunday that he had obtained the "substantial agreement of the principal parties" to a "concept and underlying plan for a possible peacekeeping operation." Though the former US secretary of state declined to elaborate on the scheme, his announcement indicated that he had persuaded Mr. Milosevic and General Kadijevic to relent in their opposition to stationing UN troops in Croatia's Serbian enclaves. Both had insisted that UN troops be deployed only in buffer positions on the boundaries of those enclaves, which would have de facto accomplished their goal of ensuring that territories wrested by force remain severed from Croatian authority. Mr. Tudjman recognized that earlier this month when he agreed to allow the stationing of international peacekeepers in all crisis areas, dropping a demand that they be positioned only on Croatia's borders with Serbia and the republic of Bosnia-Herzegovina. A second bright spot was an accord that Vance's deputy, former US Ambassador Herbert Okun, helped broker between Croatia and the Yugoslav Army to complete the removal of blockades on three military facilities in the republic's capital of Zagreb. The Serbian military high command has been adamant that Croatia lift all the sieges that it imposed in mid-September. Western diplomats have blamed Croatia's foot-dragging for the Army's violations of the Nov. 23 cease-fire, including bombardments that claimed more than 100 casualties last Friday in the southern Adriatic medieval city of Dubrovnik and the eastern town of Osijek. "I think it is of substantial importance," Vance said of the Zagreb withdrawal accord in an interview Sunday. "There has always appeared a linkage over the failure to achieve a cease-fire and the deblockation of barracks." The accord brokered by Mr. Okun and earlier agreements arranged by the Zagreb-based European Community monitoring mission required that the troops in the facilities' withdrawn entirely from Croatia. Any UN peacekeeping force deployment plan also would mandate a pullout of federal troops sent from outside the republic, and the disarming of undisciplined ultra-nationalist militias of both sides. It is in achieving this goal that Tudjman, Milosevic, and Kadijevic face their greatest challenges in cooperating with Vance. "The key to getting [UN troops] in is a workable cease-fire, and that depends on who controls the gunners in the field," says another Western diplomat. The three men have been unwilling or unable to rein in the militias, whose leaders have shown increased political ambition and substantial personal power in the territories that they control. "You have all of the local warlords, and being disarmed takes away their sole source of power," says a Western diplomat. For that reason and their rejection of any solution other than the complete destruction of their ethnic rivals, extreme Serbian and Croatian nationalist leaders have refused to turn in their weapons or endorse the cease-fire. An even graver obstacle for Milosevic and Kadijevic are objections by many Serbian political leaders and military commanders to the withdrawal of federal troops from Croatia's Serbian enclaves. Yugoslav Vice President Branko Kostic, the head of the four-man pro-Serbia bloc that seized control of the collective head of state, recently said: "The state presidency will not back a single solution which does not suit the interests of the Serbian people in Croatia." The military presents an even bigger problem. Several generals have publicly refused to withdraw their troops from Croatia's Serbian enclaves, reiterating their contention that the republic's 580,000 minority Serbs must be protected from Croatian "genocide." Analysts said that many Communist Army commanders see the creation of a new, truncated Yugoslavia through the seizure of Croatia's Serbian areas as the only way to safeguard their privileges and Marxism. "Right now, the Army is trying to devise a political entity that would preserve their privileges," said Milos Vasic, the military affairs writer for the liberal magazine Vreme. Vance admitted that because of those and other serious problems, he harbors little hope that hostilities will end. "We will have to wait and see if the fighting stops. I have no way to be sure that will be the case," he said.