Woltz, as I posted elsewhere, Vito didn't realize until after the Dons' meeting that it was Barzini all along. Sollozzo was a tool for Barzini to use to replace Vito at the apogee of the NY underworld. The drug traffic could have survived without Vito's (or Sonny's) help and, backed by Barzini, it would provide considerable income to Barzini. So, to the extent that any machinations in the underworld are personal and not business, then, yes, Barzini's efforts were personal.