Remove stems and cut each pear lengthwise in half. Wrap half of one pear in plastic wrap and place in the fridge (you will use this for garnish later). Cut the remaining pears lengthwise into 1/4-inch thick slices. Cut off the ends of the lemons and cut the lemon into quarters. Slice into 1/4-inch thick slices. Add the pears and lemons to a large pitcher.
Pour the wine and vodka (optional) over the fruit in the pitcher. Seal tightly or cover with plastic wrap and refrigerate for at least 4 hours, up to 8.
When ready to serve, strain the sangria mixture through a sieve and into a second large pitcher. Add the pear slices back into the pitcher and the lemon slices if desired. Discard any other sediment. Top off the sangria with sparkling water. Stir gently to combine. Taste and add more if desired.
Take the pear half you set aside earlier and cut lengthwise into 1/2-inch thick slices. Pour sangria into wine glasses, garnish with the pear slices and serve.