Vincent and Joseph have an odd relationship. The reason they are friends is explained in the third book, and in the short story this trilogy was born from. Though they are friends, and Joseph has pledged his life to Vincent, I feel that Joseph is not entirely happy with his decision. Therefore, he does a few things to show his independence.
From Protector of the Grey House:
The two left without a word, having been through the dance before. Joseph stepped out of the corner he was almost hiding in and poured himself a glass of brandy.
“You never ask my permission.” Vincent accepted the glass Joseph handed him.
“You have a case of this in your cellar, which I gave to you.”