While Vito did not 'order' the killing of Carlo (perhaps in part not wishing to make his only daughter a widow...NAAAHHHH), Vito was perfectly willing to allow Michael to have it done, along , it's a pretty fair assumption that he had no problem with it happening...

There was no hurry to do away with Carlo as long as he was kept under tight reigns after the 'peace' was made that allowed Michael's return. To have him alone done away with prior to the 'baptism day massacre' would've immediately arouses suspicion among the other Families, particularly Barzini.

Anyway it was imperative to get Carlo to admit to his part in Sonny's murder...something he might never have done prior to Vito's death and the settling of all 'Family Business'.

Apple