Sarah Vaughan, whose vocal equipment and virtuosity rank with the great voices of this century, opened her sold-out week at Charlie's last night with a song that employed, in part, scatted frivolity in little girl squeaks. She closed the set with a soaring "Send in the Clowns," which made majestic use of her four-octave range and her nonpareil emotive powers.

Remarking on the Washington weather of the moment, Vaughan said, "It's hot up here." It was hot up there on the bandstand, what with a double-time "Fascinating Rhythm" and an "I've Got the World on a String" that was hammered out with the relentless swing of, say, Lionel Hampton's mallets.

There was also a medley of ballads that touched the heartstrings but was not without self-conscious, humorous asides. It included "Embraceable You" and "Someone to Watch Over Me."

Throughout, Vaughan used such devices as a cappella breaks, octave leaps and saxophonelike vocal phrasing. For the male part of "Misty," her voice assumed a muscular timbre and mannish swagger. On another tune she faded herself and her regular working trio into thin air.

That trio, by the way, was so in sync with her that the total effect was that of a quartet rather than a singer with accompanists. They were pianist Mike Wofford, bassist Andy Simpkins and drummer Harold Jones. Vaughan and trio stay through Sunday.