Halve papaya; scoop out seeds, reserving 2 tablespoons. Peel and dice papaya; set aside.
Place reserved papaya seeds, papaya juice, and next 7 ingredients in a blender. Blend for 1 minute. With blender running, slowly add olive oil; blend for 30 seconds. Scoop out vanilla bean seeds, and add to mixture; stir. Add vanilla bean.
Place scallops in a large stainless steel or glass bowl; stir in reserved papaya, jalapeño, onion, and, if desired, truffle oil. Add blended mixture, stirring well.
Cover and refrigerate at least 2 hours. Remove vanilla bean before serving. Spoon into individual glasses; garnish, if desired.
Note: If using large sea scallops, cut in half. Papaya and guava juices are available in ethnic markets or on the international aisle at many grocery stores.