Boil a large pot of water.
Make poaching liquid: Whisk together Riesling, 1/2 cup sugar, the vanilla bean, and 1 cup water in a medium saucepan. Bring to a boil over high heat. Reduce heat to very low and keep it there while proceeding with step
Blanch peaches in the pot of boiling water, then peel (see "Peach-Peeling Secret," below). Halve and pit peaches.
Ease peaches into poaching liquid. Cook over high heat until liquid boils, then quickly turn off heat and cover pot with plastic wrap to seal. If edge comes unsealed, lay another piece of plastic wrap over to reseal. Set aside until cool.
Transfer peaches to a bowl using a slotted spoon. Boil liquid over high heat to reduce by half. Cool completely, then pour over peaches and chill until cold, about 1 hour.
Make topping: Whisk together crème fraîche and remaining 2 tbsp. sugar.
Place a peach half and about 1/4 cup poaching liquid in each bowl. Dollop with crème fraîche.
Make ahead: Prepare through step 4 and chill, covered, up to 1 day ahead.
Peach-peeling secret: Fuzz is a beloved characteristic of the peach, but when cooked, it can become tough. Remove the skin easily by scoring an X into the top and bottom of the fruit with a paring knife. Ease it into a pot of boiling water for 20 seconds, then transfer to a bowl of ice water. Rub skin at the X to loosen; then rub it off.
Note: Nutritional analysis is per serving.