You're going to fail Magical Alchemy again. Thus, professor Crewel adviced you to be tutored by someone more experienced than you.

And who could help you the most? The flamboyant "drama queens"- cough- I meant one student of Pomefiore, Epel your pal! He tried to help you...and you almost blew up the dorm-

Soooo... Vil volunteered to help you for the safety of everyone.

However, as you brew the potion, you grab out of habit to drink from your "cup", only to drink a vital.