Poach the pears: Using a sharp paring knife, peel the pears and give the bottoms a trim so they sit flat in a casserole or stockpot big enough to accommodate them without too much extra space. Add the wine, water, maple syrup, honey, vinegar, and juniper berries. Add a bit more water, if necessary, so that the liquid comes up to the top of the pears; it doesn’t need to cover the stems. Place the pan over medium heat and cook at a bare simmer for about 20 minutes for Anjou pears or 30 or more minutes for Bosc pears, until soft. The traditional technique is to cut a piece of parchment in a round shape so that it can sit just on top of the pears as they simmer to keep them from browning and to help them evenly poach. Or you can simply baste the pears with the poaching liquid as they simmer.
Using a slotted spoon, transfer the pears to a bowl and set aside. Bring the poaching liquid in the pan to a boil and cook until the volume is reduced by a third and it reaches the consistency of syrup.
Meanwhile, using a melon baller or a sharp paring knife, remove the lower part of the core and the seeds by making a cavity in the bottom of the pear.
Place the cored pears in a container with a tight-fitting lid. Pour the syrup gently over the pears, cover, and refrigerate for at least six hours or overnight.
When ready to serve, remove the pears from the liquid and set them on a plate to dry slightly.
Make the vanilla mascarpone filling: Meanwhile, combine the cheese, and sugar in a bowl and stir until well blended. Spoon just enough of the mixture into the hollowed core of each pear to fill the space.