How do you get a violist to play with vibrato?

Nationality: American
Age: 58
Occupation: Musician/ Composer
Residence: New York
Primary Language: English

Context: This joke was told to over the phone by an older relative of mine

Background: This relative has been teaching music for 15+ years at a small liberal arts college.

Q: How do you get a violist to play with vibrato?
A: You write the word ‘Solo’ above the notes

This joke plays on the idea that Violists are very timid, and generally try to avoid solos.