Many years ago I took a cake decorating class with my sister and the instructor had us by clear imitation vanilla in order to keep the buttercream pure white (we also had to use lots of shortening to keep it white too- yuck!). Unfortunately, everything about that buttercream was fake tasting; soon after I decided I didn't mind the slightly beige-y color my buttercream took on because the flavor of pure vanilla should never be substituted with imitation.
Any brands of vanilla you love? I know vanilla from Mexico is also supposed to be popular but I haven't personally used it. Nielsen-Massey also makes vanilla paste, vanilla beans and vanilla powder. I've used the paste and beans before- they're especially nice in a custard or ice cream where you can see the tiny vanilla flecks.