When dealing with a strongheaded person it is sometimes necessary to bait them and speak to in a language they understand.

Michael knew that to try to force Vincent to keep his fly closed would be nearly as impossible as Sonny. So ... he turns the problem into an issue of a loyal sacrifice.

He knows Vincent wants the chair and why not solve two dilemnas at once. [1] Turn his attention away from his daughter and [2] find a capable replacement.

Of course we have no way of knowing what was realy going on in Michael's head or Mary's bed.