Reduce the speed to low and gradually add the powdered sugar. Once all of the powdered sugar is incorporated, add lemon juice, one tablespoon at a time, till icing is soft and fluffy. Add vanilla and beat at a low speed until incorporated, then increase to medium-high and beat for another five minutes. If icing is too thin, add a bit more powdered sugar, if too thick, add a bit more lemon juice or milk.