some of us, who were around at the time the albums by Ben, Roy, Tatum, Carter, Hodges etc came out, were immensely grateful to Granz for recording these guys at a time when they were at their most mature, artistically. There were occasional sets that were disappointing like 'Ben Webster and Associates' but the Hawk/Ben, Roy/Tatum, Diz Afro Suite, Hodges Side by Side and Back to Back to name just a few, are musical treasures that would never have seen the light of day if not for Granz.
Re John Hammond and Vanguard - some of the early Vanguards, particularly the first Jimmy Rushing have very poor sound quality and a friend who worked with Jon Hammond said that the latter's contribution to any record session he 'produced' was to sit in the booth reading the racing pages.