Preheat oven to 325 degrees. Spray bundt cake pan with baking spray/flour.
In a large bowl, beat butter, shortening, and sugar at medium speed with electric mixer, until light and fluffy. Add eggs, one at a time, beating well after each addition.
With a wooden spoon, add flour in 4 parts, alternating with buttermilk, beginning and ending with flour. Stir to combine.
Add rum and vanilla, stirring to combine. Place batter in pan.
Bake one hour and 20 minutes. After 40 minutes cover with foil. Let cool in pan 15 minutes.
While cake is cooling, combine sugar and water for glaze in a small saucepan. Bring to a boil, stirring until sugar dissolves. Remove from heat and add rum and vanilla, let cool slightly.
Remove cake from pan. Brush glaze on cake, letting it soak onto cake after each application, until all glaze is used. Let cool completely.
Enjoy now or wrap as airtight as you can. Let cake mellow for 6 weeks or so in a very cool, dry place. If you make the cake in November, it should be just great by Christmas.