"I still am tempted to say, will now dare to say, that Sarah Vaughan is quite simply the greatest vocal artist of our century," said composer-conductor Gunther Schuller last night in the first Frank Nelson Doubleday lecture of the season at the National Museum of American History.

Then he added: "Perhaps I should qualify that by saying the most creative vocal artist of our time."

Either way it amounts to splendid praise from a man whose works are in the repertoires of the world's greatest orchestras, who has conducted most of the leading symphony orchestras and who once headed the New England Conservatory of Music.

Then, as if on cue, Vaughn, looking regal in a three-tiered, uneven hemlined, multi-shaded green dress, walked out and demonstrated everything Schuller had described.

She had the 700-person black-tie audience in the palm of her hand, performing a varied program of old standards like "Fascinating Rhythm" and "Easy Living" and newer pieces like "Send in the Clowns."

"It's wild," exclaimed Schuller after her performance. "I didn't say a word to her. She doesn't like to talk about her art. She doesn't like to analyze music. She does things intuitively. She just went out and did it."

Indeed she did. Vaughan stretched melody on "Easy Living," leaping from high notes to low ones in a single bound, all the while altering tempos. "Send in the Clowns" became a tour de force to which she applied her pliable voice, sometimes reaching a whispher, other times booming forcefully and at the conclusion constructing a quasi-operatic serpentine melody that lifted the audience to its feet for a standing ovation.

Vaughan, who got her start at an amateur show at the Apollo Theater in Harlem in 1939, wasn't at all intimidated by the august setting of the Smithsonian. Performing in the Flag Room and standing in front of the 15-star flag that inspired Francis Scott Key to write the "Star Spangled Banner" at Fort McHenry, she laughed, "I've finally made it to the Smithsonian Institution. I've worked for this." The audience loved it.

In many respects, she did her nightclub act for the. After introducing her accompanying musicians, she announced herself as "June Carter," a gag she's been pulling for years. She also offered some comic relief about the singing of drummer Grady Tate, who used to perform with her. She even imitated his way of singing "Misty" in puffed-up fashion.

Former mayor and Mrs. Walter E. Washington, both longtime Vaughan fans, said the singer's performance brought back a flood of memories.

Referring to the elections, Rep. Cardiss Collins (D-Ill.) said, "I needed something like this after last night. I foresee terrible times ahead." n

In his lecture, Schuller acknowledged that he may have shocked his audience with his effusive praise of Vaughan. Then with tongue in cheek, he said he didn't want to engage in polemics, nor was he Vaughan's press agent.

After hearing the great singers for 15 years as a member of the Metropolitan Opera Orchestra, he said from Melchoir to Bjorling and DiStefano, and from Flagstad to Sayao to Albanese and Callas and others, and, moreover, loving the music they made, "I have never found anyone with the kind of total command of all aspects of their craft and art that Sarah Vaughan has."