A vocation was one's chosen career.
Doctor Noonien Soong was a noted cyberneticist who was responsible for the construction of Data. When the two met in 2367, Data inquired of Soong as to which vocation he should have chosen to have met with Soong's approval, to which Soong replied that he had hoped that Data might have followed in his footsteps and become a cyberneticist. (TNG: "Brothers")