Vaccaro was born in Brooklyn, New York, to Italian American parents Christine M. and Mario A. Vaccaro, a restaurateur. She was raised in Dallas, Texas, where her parents co-founded Mario's Restaurant, in 1943, converting it from Dutch's Café, and where Vaccaro graduated from Thomas Jefferson High School.
She returned to New York City to study acting under the guidance of Sanford Meisner at the Neighborhood Playhouse, and made her Broadway debut in the short-lived 1961 comedy Everybody Loves Opal, for which she won a Theatre World Award.
Vaccaro's Broadway credits include The Affair (1962), Cactus Flower (1965), How Now, Dow Jones (1967), The Goodbye People (1968), the female version of The Odd Couple, (1985), and Jake's Women (1992). The husky-voiced actress is a three-time Tony Award nominee, for Best Featured Actress in a Play (Cactus Flower), Best Actress in a Musical (Dow Jones), and Best Actress in a Play (The Goodbye People).
Vaccaro appeared with Dustin Hoffman and Jon Voight in the 1969 film Midnight Cowboy, for which she was nominated for a Golden Globe Award for Best Supporting Actress. For her performance in the 1975 film adaptation of Jacqueline Susann's Once Is Not Enough she gained an Academy Award nomination and won the Golden Globe for Best Supporting Actress.
Additional screen credits include Airport '77, Capricorn One, The Pride of Jesse Hallam, Supergirl, The Mirror Has Two Faces, Heart of Midnight, Zorro, The Gay Blade and House by the Lake, also known as Death Weekend.
Her television credits include the title role in the 1976 series Sara, a number of television movies, and a regular role in the short-lived 1984 series Paper Dolls, in addition to guest appearances on Banacek, The Fugitive, The Defenders, Coronet Blue, The Name of the Game, Marcus Welby, M.D., McCloud, The Streets of San Francisco, The Love Boat, St. Elsewhere, Murder, She Wrote, The Golden Girls, Columbo, Touched by an Angel, Friends, The King of Queens, and Nip/Tuck. She was nominated for an Emmy Award three times and won for Best Supporting Actress in Comedy-Variety, Variety or Music for The Shape of Things in 1974.
She supplied the voice for Johnny Bravo's mother Bunny Bravo in the animated cartoon series. She was the first voice of Jay's (Jon Lovitz)'s ex-wife Ardeth on The Critic. She made an appearance on The Smurfs as Scruple, an apprentice of Gargamel, opposite Paul Winchell.
After ill health forced Valerie Harper to bow out of the production of Nice Work if You Can Get It at the Ogunquit Playhouse (Maine), Vaccaro took over the role of Millicent Winter for the remaining performances of the limited run from August 4-15, 2015.