Organized crime organizations have a plethora of ways to disguise financial transactions. Puzo may have actually intended that the property was sold legitimately to underscore Vito's acumen as he expressed in the novel. But we are allowed to speculate and, in light of the unplanned GFII, we speculate like crazy.

Clemenza's family would not need all that room or upkeep expense. Anything's possible. His wife might have died before him, etc. the timeline is so screwed up in GFII, it's nearly impossible to make any logical sense of it.