Using a very sharp 5-hole zester, remove zest from 6 of the oranges. Bring 1 cup water to a boil in a small saucepan, add zest, and boil 30 seconds. Pour zest through a strainer into a bowl and return orange water to saucepan. Rinse zest with cold water and set aside.
Add sugar to orange water and bring to a simmer over medium-high heat. Lower heat to medium (adjust to maintain an active simmer) and add cinnamon sticks, cloves, star anise, and vanilla bean. Simmer 10 minutes. Add zest, remove from heat, and let cool 30 minutes.
Meanwhile, cut a thin slice from each orange bottom so oranges stand upright on your cutting board. With a very sharp, smooth-bladed paring knife, slice off peels and thick white pith from oranges. Cut oranges crosswise into 1/4-in.-thick slices, removing seeds and discarding any fibrous ends.
Remove spices from syrup (reserve for garnish, if you like) and add lemon juice. Cover bottom of a large, shallow glass serving dish (13- by 9-in. is ideal) with a layer of orange slices and sprinkle on 1 1/2 to 2 tbsp. of syrup and several pomegranate seeds. Repeat layering with oranges, syrup, and pomegranate seeds until all oranges are used. Pour any remaining syrup over oranges. Just before serving, top with prosecco and garnish with reserved whole spices, if you like.