the reason why you get a sheen is because of its passive when you use an ability your basic attack will do more damage and your vorpal blade has a 2 second cooldown so you can spam it.

remember that when you use the vorpal blade on a minion or a champ try to hit them again with your basic attack so you can heal you will see a sword over their head so that mean when you hit whatever with your basic attack you will start healing a bite.