Combine sugar, milk, and butter in a medium Dutch oven. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches soft ball stage (234°).
Remove from heat (do not stir). Cool to lukewarm (110°). Add vanilla, walnuts, and raisins; beat at medium speed of an electric mixer until mixture thickens and begins to lose its gloss. Working rapidly, spread mixture in a buttered 8- inch square baking pan. Mark top of warm candy into 1-inch squares, using a sharp knife. Cool completely before cutting.