1. Heat a grill to medium (350° to 450°). With a sharp knife, halve chicken pieces horizontally almost all the way through, then open up like a book. Put chicken in a bowl and add 2 tbsp. hoisin, the five-spice, soy sauce, and 1 tbsp. oil, tossing to coat.
2. Grill chicken, turning once, until cooked through, about 10 minutes. Let rest 10 minutes, then slice diagonally into wide chunks.
3. Mix remaining 2 tbsp. hoisin and 1 tbsp. oil, the lime juice, and fish sauce in a small bowl; set aside.
4. Spread the bottom of the cut side of each roll with 1 tbsp. mayo. Fill sandwiches with chicken. Top each with cucumber, carrot, jicama, chile slices, and herbs. Drizzle each sandwich with some of the reserved lime dressing (you may have some left over).