SPECIAL United Nations envoy Cyrus Vance left Yugoslavia after a week of shuttling between Serbia and Croatia. He had a plan for using UN peacekeepers to keep the warring sides apart, but no way of using it. On Dec. 6, another cease-fire fell through, and without an established cease-fire, UN deployment is impractical.At this point, the outlook for a permanent end to the fighting is dim. A major problem is the relative autonomy of the military forces involved. The Yugoslav Federal Army is a fighting force without a country. "Federal" oversight now resides in the expansionist Serbian government. The pro-Army local Serbian militias in Croatia operate pretty much as they please, breaking cease-fires if it suits them. Behind the vicious combat is the engrained hatred between Croats and Serbs. The war has produced some 400,000 refugees; many have stories of brutality that rival those of World War II, when the Croats and Serbs butchered each other. But ethnic animosity isn't the only motivation at work in Yugoslavia. The country's breakup is now a certainty, and the republics are to some extent competing for its assets. The parts of Croatia currently occupied by federal and Serbian forces include much of its most productive agricultural land, as well as oil resources. The Dalmatian coast, under siege by the federal Navy and troops, is a crucial asset because of tourism. The UN and the European Community are devising economic sanctions to bring an end to the fighting, and the US has imposed a trade embargo. It might be useful to make it clear, as well, that future aid will be tied to such steps as clear respect for minority rights and reduced military expenditures. When the fighting does stop, the republics face a huge rebuilding job. The longer the war, the harder that job becomes.