Vacuum-Macerate the Peaches: Place the sliced peaches, granulated sugar, and the optional vanilla into a chamber or suction vacuum sealer bag. Seal at high vacuum. Massage the bag gently to coat the fruit in sugar, then place in the refrigerator for at least 4 to 12 hours until the sugar completely dissolves into a clear, vibrant peach syrup. Cut open the bag and strain the syrup through a fine-mesh sieve (save the sugary peach slices for garnish or topping ice cream!).
Release the Mint Aromatics: Place fresh mint leaves in the palm of your hand and give them one hard slap to release the essential oils. Drop the mint into your julep cup or glass.
Build the Drink Base: Pour in the peach syrup and fresh lemon or lime juice over the mint.
Pack with Ice & Swizzle: Fill the glass halfway with crushed ice. Use a long bar spoon to swizzle vigorously for 15 seconds. Top off with more crushed ice and sparkling water.
Garnish & Serve: Garnish with a fresh peach slice and a generous sprig of mint slapped over the back of your hand before placing it next to the straw. Serve frost-cold!