Ah, Dunkaroos! I remember begging my mom for them when I was a kid! According to a variety of sources, this should do the trick:
1/2 cup icing sugar
1 T milk (substitute)
1/4 tsp. Vanilla extract
There are other recipes out there that I'm sure are delicious (a number of them call for funfetti cake mix, which I'm not sure is vegan- you are supposed to mix it with yogurt and cool whip, and by the time you'd made all the vegan substitutions, I'm not sure it would be very good).
You can also always go the easy route and buy Duncan Hines vanilla frosting, which is usually vegan.