Some of the 75 Vietnamese who fled their communist homeland in search of a better life wave in Hong Kong Thursday before boarding a chartered jet for Hanoi in the first large-scale repatriation of boat people. They were among 11,000 Vietnamese who have sailed into Hong Kong since it adopted a tough new policy last year that for most of them rules out resettlement. When they touched down in Hanoi, they were met by Vietnamese, U.N. and British officials, Hong Kong radio reported. Hanoi has promised not to prosecute the returnees and to allow the United Nations to monitor their treatment back home. The repatriation was arranged under an agreement reached by Vietnam and the U.N. High Commissioner for Refugees.