Preheat oven to 325°F. Using nonstick cooking spray, lightly grease then flour a 10-inch Bundt pan. Set aside. In an electric stand mixer, beat butter and shortening at medium speed until creamy. Gradually add granulated sugar, beating until light and fluffy. Add eggs, 1 at a time, beating until just blended after each addition. In a large mixing bowl, stir together flour, baking powder and salt. Add flour mixture to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed until just blended after each addition. Stir in vanilla and rum. Transfer 2½ cups batter to a large mixing bowl then mix in cocoa and red food coloring. Using a small cookie scoop, drop 2 scoops of plain batter into prepared pan. Top with 1 scoop red velvet batter. Repeat around entire pan, covering bottom completely. Continue layering batters in pan in the same fashion until all batter is used. Bake 1 hour or until a wooden pick inserted in center of cake comes out clean. Remove from oven and cool on a wire rack 10 minutes. Remove cake from pan and place directly on wire rack to cool completely, approximately 1 hour. While cake is cooling, make vanilla glaze. In a medium mixing bowl, whisk together all ingredients for vanilla glaze until smooth. Once cake is cooled completely, drizzle with glaze and serve.