HONG Kong is expected to launch mandatory repatriation of Vietnamese boat people detained in camps here as early as this month, Western diplomats say.The decision to revive the policy of forced repatriations follows an accord struck by Britain, Hong Kong, and Vietnam in Hanoi Sept. 27 on "accelerating the rate of return" of nearly 20,000 Vietnamese who have not qualified as refugees. A key factor in the decision was Vietnam's agreement in principle to allow nonvoluntary repatriation, Hong Kong officials say. Hanoi earlier sided with Washington in opposing forcible returns. "These people have to go back [to Vietnam]. We're talking about a major logistical exercise," says Paul Brown, a Hong Kong government spokesman for refugee affairs. Hong Kong and Hanoi are still working out the details of how to transport the boat people and ensure adequate facilities for them upon their return, he adds. Western diplomats say the forced repatriations are almost certain to begin this month. Aircraft would be "the quickest way, but also the most dangerous," to transport the boat people back to Vietnam, one diplomat said. Some 140 boat people held here at the Nei Kwu Chau detention center will likely be sent first. They had volunteered to go back to Vietnam but later returned. The atmosphere remained quiet yesterday in Hong Kong detention centers where nearly 60,000 Vietnamese boat people are held in cramped, jail-like conditions. More than 19,000 of the boat people have been labeled "illegal immigrants" by Hong Kong; the rest await screening. Hong Kong's only attempt at mandatory repatriation was in December 1989, when the forced return of more than 50 Vietnamese sparked protests in camps and an international outcry.