Combine first 3 ingredients in a medium saucepan. Cook over medium heat 5 minutes, stirring constantly, or until sugar dissolves; pour evenly into 2 lightly greased 9-inch round cakepans. Sprinkle 3/4 cup pecans evenly over brown sugar mixture in pans.
Beat granulated sugar and oil at
medium speed with an electric mixer until smooth. Add eggs, 1 at a time, beating until blended. Add pumpkin and 1/4 teaspoon vanilla, beating just until blended.
Combine flour and next 4 ingredients; add to pumpkin mixture, beating just until blended. Spoon over pecans in pans. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 5 minutes. Carefully loosen cake from edges of pan (do not tear pecans from cake). Remove cake from pans, and cool completely on wire racks.
Beat 1 3/4 cups whipping cream at medium speed until frothy. Add powdered sugar and 1/4 teaspoon vanilla; beat at medium-high speed until soft peaks form.
Place 1 cake layer, praline side up, on a cake plate. Spread half of whipped cream over cake layer; top with remaining cake layer, praline side up. Spread remaining whipped cream over cake layer. Sprinkle with 1/4 cup toasted pecans. Cover and chill. Store in refrigerator.