President Reagan will increase this fiscal year's quota of Soviet and Eastern European refugees by 5,500 by lowering the quota of Vietnamese refugees, many of whom have been unable to emigrate, a senior U.S. official says.

The decision brings to 30,000 the number of Soviets who will be permitted to enter this country. The added slots will be taken from the 25,000 ceiling for refugees from Vietnam, but the official said the president has considerable flexibility to alter the totals later on.Thousands of Soviet refugees, many of them Jews, are backed up in Vienna and Rome, waiting for permission to come to the United States, and the quota for them to enter this country is full.

Many of the Vietnamese who were expected to come here this year have not been cleared by Hanoi authorities. A number of those Vietnamese are former political prisoners.

The Reagan administration, after pressuring the Soviet Union to step up Jewish emigration, has found itself in a paradoxical position.

The Jewish exodus is at a nine-year high but the United States is now unable to process all those who left the Soviet Union with exit permits for Israel, who then sought entry here after reaching Vienna and Rome.