Unocal Unit Signs Vietnam Natural Gas Pact: A subsidiary of El Segundo-based Unocal Corp., Repsol and a unit of Mitsui Oil Exploration Co. signed a three-year contract to explore for natural gas off the coast of Vietnam. Unocal Vietnam Exploration Ltd. will take a 45% stake in the contract to search for gas in the northern part of the Malay Basin. The Spanish oil and gas company Repsol will take a 30% share and Moeco Vietnam Petroleum Co., a subsidiary of Japan's Mitsui Oil Exploration, will take a 25% share. The signing of the contract comes after two years of negotiations between Unocal and PetroVietnam, the state-run oil company. PetroVietnam retains an option to acquire a 15% interest in the project, which would reduce the other partners' shares proportionately. Unocal said in its statement that a first exploration well will be drilled in 1997.