Look for brightly colored cara cara and blood oranges, which are available this time of year.
Whisk together first 3 ingredients in a medium bowl. Split vanilla bean in half lengthwise; scrape seeds with the back of a paring knife into oil mixture. Whisk to combine. Let stand at room temperature for 1 hour. Whisk in salt and pepper.
Cut away rind and white pith from oranges and grapefruit. Cut each into 1/4-inch-thick rounds.
Combine watercress and 1 tablespoon dressing in a large bowl; toss to coat. Arrange on a platter, reserving a few leaves. Top greens evenly with citrus rounds, radish slices, and reserved watercress leaves. Serve with remaining dressing.