Working my way with The dark brotherhood, and now I am on a quest to kill every brotherhood member (purification). Problem is I wanted to try vampirism before that, and Vicente has never offered me dark gift option. I did his quests with optional sides, and received some nice items but when I talk to him about vampirism he just says "When I trust you more blablabla". I have his disposition at 100, so he should trust me.

What did I miss? Is there a console command to start that conversation?

I'm not sure why he's not offering you the gift, but you don't need to get it from him. Just go to any vampire lair, get hit a few times, then kill them or flee. You get nothing unique from him.

Easiest vampire to face is Count Hassildor. Provoke him to punch you a few times. He can't hurt you much.