1. In a medium bowl, combine the vinegar with 1/2 cup water. Add the onions, making sure they’re fully submerged (if not, add a splash more vinegar). Set aside for 30 minutes.
2. Meanwhile, in a small bowl, stir together the mayo and sriracha, tasting and adjusting the spice level to your preference.
3. Cut the baguette into 4 equal pieces. Then cut lengthwise to make a slit, still keeping one side intact. Pull out some of the fluffy interior to make space for the fillings (this is a chef snack!). Set aside.
4. Heat the oil in a large non-stick skillet over medium-low heat. Halve the hot dogs lengthwise and place in the skillet in a single layer. Cook until golden all over, about 3 minutes per side.
5. To assemble, spread some of the mayo on the bread and pile in the cucumbers, hot dogs, pickled onions, and cilantro. Serve immediately.