Adding in the vanilla is a must if you are using homemade peanut butter for the recipe. Most store-bought versions have their own mild flavorings added in, which can effectively mask the raw, oily flavor and smell that the peanut butter has, but homemade peanut butter doesn’t.  The vanilla can mask those flavors, and also lend your candy a nice, subtle, and classic fragrance that doesn’t overpower the flavor of the peanuts.