put the sugar, shortening, butter, and vanilla in a bowl of a stand mixer fitted with the paddle attachment, and paddle on low-med speed into mixture is smooth, with no lumps, this will take about 3 minutes,( if you only have a hand mixer, start mixing on low to medium, until you get mixture smooth with no lumps)

with motor running add both amounts of water in a thin stream and continue to paddle until all water is absorbed( for a hand mixer, if you cannot add in a stream add 1 teaspoon at a time and mix in)