Combine vinegar, sugar, and salt in saucepan; bring to boil over medium-high heat. Stir to dissolve sugar (1 minute); let stand until cool (2030 minutes). Add carrot and radish. Let stand until vegetables pickle (15 minutes); strain and discard liquid. In bowl, mix Sriracha and mayonnaise. Slice half of avocado; set aside. Spread rest of avocado on half of toasted baguette. On remaining half baguette, spread mayo mixture. Fill sandwich with avocado slices, pickled vegetables, sliced jalapeño, and cilantro leaves. Cut into 3 pieces.