[1] After that, until 1974, he was a member of pianist George Shearing's group, and from 1979 to 1989 toured with singer Sarah Vaughan.
Throughout and after that time, during which he settled in Los Angeles, Simpkins became respected as a top-quality bassist and widely known as a solid and reliable studio musician.
[1] He performed with singers Carmen McRae and Anita O'Day, instrumentalists Gerald Wiggins, Monty Alexander, Buddy DeFranco, Don Menza, and Stéphane Grappelli, and many others.
[1] He also played acoustic bass on the 1997 covers album In a Metal Mood: No More Mr. Nice Guy by artist Pat Boone.
[2] With The Three Sounds With Pat Boone With Kenny Burrell With Benny Carter With Natalie Cole With Teddy Edwards With Victor Feldman With Barbara Morrison With Robert Palmer With Lalo Schifrin With George Shearing and Stéphane Grappelli With Mary Stallings With The Temptations With Toni Tennille With Sarah Vaughan With Joe Williams and George Shearing