Combine first 6 ingredients in a medium bowl. Combine vinegar and next 3 ingredients, stirring well with a whisk. Add vinegar mixture to tuna mixture, tossing to coat.
. Cut a "V"-shaped pocket in top of baguette; remove cut portion, and hollow out inside of bread, leaving a 1/2-inch border. Reserve cut portion and torn bread for another use.
. Spoon tuna mixture into hollowed-out baguette, mounding slightly. Cut filled baguette crosswise into 4 equal portions.