Only a few of the modern-dance faithful were on hand at Saturday night's $25-a-ticket benefit by and for Jan van Dyke and Dancers. John Mondale contribute her benevolent presence to the event at Trapier Theater: Benetta Washington and Dorothy Fauntroy lent their names: and Patrick Hayes, Washington's white-haired Santa Claus of the arts, blessed the occasion with a few graceful words on the importance of governmental support for the artists.

Van Dyke best known of Washington's modern dancers, opened with her solo "Waltz," a piece which grows more beautiful with every viewing. Perverse and lyrical, "Waltz" is so nearly perfect in gesture and elegant in concept that it tends to dwarf much of the subsequent work.

"The Story of Twilight . . ." looked better on the small Trapier stage than it did at Lisner in April. Van Dyke has made changes which have improved this surreal fairy tale of transformations, but further pruning is probably needed for the rythm and succession of images to come clear.

At her most ingratiating, Van Dyke conveys a sense of the sheer pleasure of dancing which can get lost in her heavier, more surrealistic works. In "Good-bye, Elvis.," however, premiered to music of Fleetwood Mac, she has created a lovely blend of popular and balletic movement which manage easy transitions from style to style without compromising the lyrical tone of the whole or falling into parody and slapstick.

Friday night's "Concert for the Maple Trees," by Little Sister Tidepool (Penelope Newcomb of Vermont), was cancelled because of illness, leaving the Washington Project for the Arts with a 600-pound barrel of maple syrup to store indefinitely. (The syrup was to have been sold after the performance on a bring-your-own-container basis). W.P.A. hopes to reschedule the event when the artist recovers.