ALL 40,000 Vietnamese boat people in Hongkong's detention camps should be repatriated within three years, according to the Secretary for Security, Mr Alistair Asprey. The United Nations High Commissioner for Refugees (UNHCR) has stepped up its efforts to promote voluntary repatriation recently and further initiatives were planned. ''The population . . . is about 40,000. On current trends, they should have returned to Vietnam within three years,'' Mr Asprey said. Replying to a question from Mr Martin Barrow, Mr Asprey said more than 28,000 Vietnamese had already gone home under the voluntary repatriation programme - an average return rate of 1,000 per month - since Hongkong struck a deal with Hanoi in October 1991. Mr Asprey said that 1,500 per month was a sustainable figure and added that of all migrants who returned to Vietnam from the Asia region, Hongkong had probably sent back about two-thirds of them. UNHCR efforts include photographic exhibitions on Vietnam, the screening of Vietnamese television programmes, bi-monthly Vietnamese language bulletins on developments back home and regular reports on UNHCR monitoring of those who have already returned. Mr Asprey said the Government had recently converted Section 9 of the Whitehead detention centre into a voluntary repatriation centre with a capacity for more than 2,500 residents. ''We are now converting Section 10 of Whitehead into another centre with a similar capacity,'' he said. Mr Asprey reiterated the British Government's support for the lifting of the American trade embargo on Vietnam and access by the Vietnamese Government to international funding through the International Monetary Fund and the World Bank.