Reduce speed to medium. Add sugar, 1/2 cup at a time, beating after each addition, about 5 minutes. (After every 2 additions, increase speed to high, and beat for 10 seconds, then reduce speed to medium-high). Add vanilla, and beat until buttercream is smooth. Use immediately, or cover and refrigerate for up to 3 days. (Bring to room temperature, and beat on low speed until smooth before using.).