MOSCOW, March 23 : Russia's largest liquefied natural gas producer Novatek has signed a preliminary contract with a buyer from Vietnam and is ready to begin supplies in the near future, company's chief executive Leonid Mikhelson told Russian state TV on Monday. • Vietnamese Prime Minister Pham Minh Chinh is on four-day visit to Moscow. • Novatek has been holding negotiations with buyers for more than five years, and a preliminary supply agreement was signed with one of them quite recently,