Prodigies tend to fade away, but Saturday night in the Kennedy Center Concert Hall, Marvin Hamlisch proved one of the few who flourish. A Juilliard student at age 7, Hamlisch is now 33 and a thriving composer ("A Chorus Line" and 'They Way We Were'), arranger ('The Sting'), pianist, conductor and compere.

For his Washington bow, Hamlisch enjoyed the collaboration of three vocalists - Lonnie Ackerman, Valerie Lemon and Sandra Voris - and a symphony orchestra of 80 pieces, led, when Hamlisch was at the piano, by "A Chorus Line" conductor Don Pippin.

Richness of sound was the chief pleasure of the evening (benefiting B'nai B'rith Women's Silver Chapter and the National Jewsish Hospital) in a program drawn from Gershwin and Joplin as well as Hamlisch. Theater orchestras usually number 26 pieces and film scores come to us through electronics , so the luxury of pure sound and full orchestration was an uncommon experience.

Because "A Chorus Line" begins with an audition in rehearsal dressa, it has no formal overture. Hamlisch introduced a 10 minute, independent piece which merits wide usage on major symphony "pops" programs, as Gershwin, Kern, Porter, Rogers and Arlen were performed with Russell Bennett orchestrations by leading symphonies. The effect is, oddly, far richer than a full-score performance.

Hamlisch had two other novelties, moments from his score for the new James Bond film, "The Spy Who Loved Me," which seemed to capture, in an almost satirical vein, the electronic sharpness of Ian Fleming's fictional character. And to sing "What I Did for Love," he introduced a striking, elegant singer (Sandra Vormo) making a debut of the first rank. Remember the name: Sandra Voris.

The Hamlisch arrangement for some of the Gershwin "Porgy and Bess" score amounted to a piano concerto with instruments as the voices for "Summertime," "I Loves You Porgy," "I Got Plenty of Nothin'" and "My Man Is Gone Now."

It is a protean, unique performance, Hamlisch as symphonic artist, one the major orchestras will call on for years to come. And as compere, Hamlisch comes through as a genial, sincere guide, an artist who as grown from boyhood prodigy.