The Mona Lisa is currently located in the Louvre and was there since 1797.  It is the most famous and viewed painting currently and therefore the security is very good.

The Last Supper is currently located in Santa Maria delle Grazie.  The Last Supper shows Christ’s final meal with his apostles.  He says that one of them will betray him and later Judas handed Christ over to the authorities who arrest him.

The Vitruvian Man was a sketch/drawing by da Vinci who was studying the actual human proportions.  A nude male was in 2 positions and split up proportionally. Nobody commissioned this.