Aizen wants that power, but maybe Vaizard transformation requires something else (grand fisher said that being a 2nd generation shinigami gave Ichigo extreme powers, maybe that's what Aizen is lacking?) or Aizen believes that if he unlocks the hogokyou's power to the limit, he might get a new combinnation, since Ichigo vaizard mode seems still 'unperfect' (compared to the arrnacer mode)..