In a small bowl, whisk together vinegar, oil, salt, and sugar. Cut vanilla bean in half lengthwise, scrape seeds from one half with the back of a paring knife, and add seeds and scraped pod to vinaigrette (reserve other half of pod for another use). Set aside at room temperature for at least 1 hour to let the vanilla flavor develop.
Arrange lettuce leaves on a large platter in a single layer. On each leaf, lay 1 or 2 slices of papaya and 1 or 2 slices of avocado. Drizzle vinaigrette over salad.
*Available at gourmet grocery stores and from hawaiianvanilla.com
Make ahead: Mix vinaigrette up to 1 day ahead and chill.
Note: Nutritional analysis is per serving.