MIX the vinegar, sugar and salt together in a bowl. ADD the carrot, cucumber and bean sprouts, toss well to combine, then let stand for at least 20 minutes while you prepare the remaining ingredients. MASH the tuna thoroughly with a fork, then stir through the mayo, scallion and fish sauce to combine. WARM the baguette in the oven to lightly crisp the outside, then cut in half lengthwise (leaving one side of the bread as a hinge) and remove some of the soft bread from the center of each half to hollow out a little. SPREAD the tuna mix evenly on the base of the baguette, then place the lightly-pickled carrot, cucumber and bean sprout mixture over the top. PLACE the sliced jalapeño peppers then the cilantro on top. CLOSE the baguette, cut in half, and serve.