Place the vegan butter into a large bowl, or the bowl of a standing mixer. Beat on high speed for a couple of minutes, or until light, creamy, and fluffy.
Add the icing sugar to the bowl and mix on low to combine. Once the sugar is incorporated, slowly increase the speed to high and beat for another minute.
Continuing to mix, add the vanilla. Slowly add the milk while mixing on high speed, a tablespoon at a time, until the buttercream reaches the desired consistency.
Rub a little bit of the icing between your fingers - if it feels gritty, beat for a little longer. It should be very smooth and fluffy when it's ready.
Store the buttercream in a sealed container in the refrigerator for up to one week.