In a small saucepan, combine the white vinegar, water, granulated sugar, and salt. Bring to a boil, stirring until the sugar is dissolved.
Remove from heat and let it cool to room temperature.
Place the julienned carrots and daikon radish in a bowl or jar and pour the cooled pickling liquid over them. Let them marinate for at least 1 hour or overnight in the refrigerator.
Preheat the oven to 350°F (175°C).
Place the Vietnamese baguettes in the oven for about 5 minutes to warm them up and make the crust crispy.
In a small bowl, mix the mayonnaise with the soy sauce and fish sauce until well combined.
Slice the warmed baguettes lengthwise, leaving one side attached to create a hinge.
Spread a generous layer of the mayonnaise mixture on both sides of the baguette.
If using, spread a layer of liver pâté on one side of the baguette.
Layer the sliced grilled pork (or your chosen protein) on the bottom half of the baguette.
Top with pickled vegetables, sliced cucumber, jalapeño or red chili (if using), and fresh cilantro leaves.
Close the sandwich and press gently.
Enjoy your meal!