Preheat oven to 350°. Make syrup: In a bowl, whisk wine, white wine vinegar, sugar, and salt until sugar dissolves. Pour into a 9- by 13-in. glass or ceramic baking dish. Scrape seeds from vanilla pod into dish, then add pod and peppercorns.
Slice quinces crosswise into 1/4-in. pieces and add to baking dish. Cover with parchment paper cut to fit, then tightly cover with foil. Bake until quinces are tender when pierced, about 2 1/2 hours.
Strain syrup into a bowl and let cool; stir in Champagne vinegar. Transfer to a jar or bottle and chill until cold, at least 2 hours; stir before using.
For shrub, pour 2 tbsp. syrup into an 8-oz. glass filled with ice. Add sparkling water, stir, and top with a lemon twist.