Cut a shallow X in bottom of each peach with a sharp paring knife and immerse fruit in a 4-quart heavy pot (preferably wide) of boiling water 30 seconds, then transfer with a slotted spoon to a bowl of ice and cold water to stop cooking. Transfer peaches to a cutting board and peel, starting with cut end, then cut in half, discarding pits. Combine sugar and water in cleaned pot. Halve vanilla bean lengthwise with a paring knife and scrape seeds into pot, then add pod and bring mixture to a boil, stirring until sugar is dissolved. Add peaches, pitted sides down, to sugar syrup, then reduce heat and poach, covered, at a bare simmer 6 minutes. Turn peaches over and continue to poach, covered, until tender, 5 to 6 minutes more. Cool peaches in poaching liquid in pot, uncovered, 1 to 1 1/2 hours. Force raspberries through a fine-mesh sieve into a bowl, pressing hard on solids. Combine sugar and water in a small heavy saucepan. Cover pan (so condensation will wash any sugar crystals down side of pan) and bring to a boil, then remove lid and boil 2 minutes. Immediately stir sugar syrup into raspberry puree and cool to room temperature, about 1 hour. Stir in lemon juice. 3Serve peaches over ice cream and drizzled with sauce.