Combine carrots, jicama, and onion in a small bowl.
Combine 1/2 cup vinegar, water, and sugar in a saucepan over medium heat. Stir until sugar dissolves, about 1 minute. Remove from heat and pour pickling liquid over the sliced vegetables. Let sit for 30 minutes.
Whisk remaining 1/4 cup vinegar with fish sauce, soy sauce, sriracha sauce, garlic powder, five-spice powder, salt, and pepper in a 10-inch saute pan over medium heat. Bring to a boil, about 3 minutes. Add portobello mushrooms; cook until tender, about 5 minutes. Remove mushrooms from the liquid with a slotted spoon.
Spread 1 tablespoon mayonnaise over each roll half. Place 1/2 the cooked mushrooms over bottom halves. Drain pickled vegetables and place over the mushrooms. Divide cucumber, jalapeno, and cilantro between the sandwiches. Add more sriracha if desired. Squeeze 1 lime wedge over the filling of each sandwich and cover with top bun half.
