CONTINGENCY plans are being drawn up to deal with the increasing number of Vietnamese illegal immigrants coming to the territory from China. The Government was hoping to house the Vietnamese in the Kai Tak Transit Centre and Chi Ma Wan Detention Centre which have a total capacity of about 2,000. But with the present rate of arrivals, which have already reached 1,452 this month, the two camps are likely to be filled soon, while signs of early repatriation are still remote. A government source said he was not particularly encouraged by the Chinese Foreign Ministry statement saying the mainland was concerned about the influx and would take the Vietnamese back as soon as possible. ''We are confident that China will take them back eventually, but it is the timing that we are worrying about.'' What the Chinese authorities had agreed to was just the principle, and it could take a long time to implement, he said. ''Our wish is to have some of them sent back first so as to discourage further arrivals.'' It has been claimed that up to 10,000 Vietnamese in Beihai, where their homes were demolished by local officials, are preparing to set out for Hongkong. The Government is juggling with limited accommodation facilities to house the new arrivals, although there is space for another 5,000 people in other detention centres. Statistics provided by the Correctional Services Department show the total capacity of the six detention centres manned by the department is 34,824 and the population at the last count on July 17 was 29,346. But not all the available vacancies can be used to accommodate the Vietnamese. The source said one option under consideration was to partition Whitehead Detention Centre so the Vietnamese illegal immigrants from China were separated from the boat people. But there was no immediate need for this because Chi Ma Wan detention centre could still cope. Meanwhile, immigration officials also expressed concern that manpower constraints could delay plans to complete the screening process for Vietnamese boat people by the end of the year. About 50 officers, most working in the screening division, have been deployed to investigate the identities of the recent arrivals. Acting Assistant Director of Immigration Choy Ping-tai said as they did not have extra manpower to handle the new influx, the deployment would inevitably slow the screening process. But he stressed they would keep the disruption to a minimum and would send the officers back to screen boat people once the problem was settled. The United Nations High Commissioner for Refugees in Hongkong maintained the Government should keep the issues of boat people and illegal immigrants separate. The commission's deputy chief-of-mission, Kaiser Zaman, said the influx of Vietnamese illegal immigrants should not in any way affect the screening process of boat people.