"I'd sacrifice..." He fell on one knee in front of you, bowing to you, his eyes closed and his heart open, the words from his mouth leaving it with considerable difficulty. "I'd sacrifice myself to you..." 

Those were strong words, strong for someone who was perhaps showing his feelings for the first time. He needed you like he never needed anyone else, he could lie to himself, but you gave him a comfort he hadn't felt in so long and now he clings to it, like a lifeline.

Vergil is a devoted man, to his goals, his beliefs... and now, to you.