In a large saucepan, combine the sugar, Prosecco, water, vanilla bean and seeds, lemon zest, cinnamon stick half and cloves and bring to a boil. Add the pears and keep them submerged by covering them with a small lid. Reduce the heat to moderate and simmer until the pears are tender but not mushy, about 20 minutes. Using a slotted spoon, transfer the pears to a cutting board. Let cool, then thinly slice them lengthwise. Reserve the poaching liquid in the refrigerator for poaching other fruit.