Heat the milk in a medium saucepan over medium-low heat until it starts to steam. Remove one cup of milk, set it aside, and keep it warm. Stir into the larger pan of milk the remaining sugar, the vanilla bean and seeds, and the rice. Lower heat and cook, stirring constantly, until the rice is tender, about 20 minutes. Add the reserved milk, lemon zest, butter, and salt. Stir well and remove from the heat. This should have a more liquid consistency than regular risotto.