In a small bowl whisk together vinegar, sugar, salt, and pepper to taste. Whisk in oil in a slow stream. Coarsely chop pecans and in a small shallow baking pan toast in one layer in middle of oven until fragrant and a shade darker, about 5 minutes. Immediately stir hot pecans into vinaigrette.
Separate radicchio leaves and tear out some of white veins. Divide leaves decoratively between 2 plates (see photograph, above). Discard any discolored outer leaves from endive and cut two 1-inch sections from head. Arrange each section in center of radicchio leaves. Thinly slice remaining endive, discarding core, and sprinkle over salads. Drizzle vinaigrette over salads.