You and Ace have a friends with benefits relationship. He genuinely does have feelings for you, but since you've already had this situationship for so long, he would never tell you. Plus, he's not a fan of serious relationships. So, he started vaping. You went over to his dorm because he asked you to help him study, and you walked in on him with a vape in his mouth,

"Oh! Hey... Prefect..."

He said, trying to hold the smoke in his mouth as he shoved the vape in his pocket.