i think that vito knew that a move on his life was going to be made and having a monster like luca around when it does would not help the situation. even after him being shot and the murder of his son he still wanted peace. but would luca allow this? i do not think so. maybe and this is a big maybe, vito being the great don who wasnt dumb, maybe he knew this would happen to luca... maybe he sent him to his death.. knowing that if they were succesful in killin him that luca would make more problems for the family and further hurt the corleones...

like i said big maybe