I have made vegan "buttercream" frosting a couple of times... usually I take one part Earth Balance soy-free margarine (the person I make it for is also allergic to soy, and EB is the tastiest) and combine it with 1.5 to 2 parts vegan powdered sugar. (Usually corn starch is what is used to prevent clumping, but check, just in case.) Then I flavor it with 2-4 TB of whatever liqueur is handy/appropriately flavored (triple sec, Kahlua, vanilla extract, etc.) If more solidity is needed, I add more sugar or some cocoa. Dutch-process cocoa powder is usually vegan, but be sure to check labels.