When making vegan mashed potatoes, you want to keep the flavor as neutral as possible. You always want to go with an unsweetened, unflavored milk. Sugar has no place in mashed potatoes and definitely nothing flavored is needed. You also don't want it to be overly nutty as that flavor will come out into the vegan mashed potatoes. The best dairy free milk for mashed potatoes is a creamier milk like oat milk or soy milk.