Marinate the chicken: Combine the lemongrass, soy sauce, sesame oil, fish sauce, garlic, ginger, lime juice and ½ teaspoon pepper in a food processor and puree to make a coarse paste. Transfer the paste to a large resealable plastic bag, add the chicken, squeeze out the air and seal. Gently move the chicken around inside the bag to coat. Refrigerate at least 30 minutes or up to 2 hours.
Marinate the vegetables: Whisk the vinegar, sesame oil, honey and ½ teaspoon salt in a medium bowl. Add the carrots, radishes and jalapenos and stir to coat. Cover with plastic wrap and refrigerate 20 minutes.
Make the tacos: Brush a large grill pan with vegetable oil and heat over medium-high heat. Remove the chicken from the marinade and grill until just cooked through, 5 to 8 minutes per side. Transfer to a cutting board; slice the chicken against the grain and brush with the hoisin sauce.
Combine the sour cream, lime juice, Sriracha and scallions in a blender; blend until smooth. Heat the tortillas in a dry skillet or in the microwave. Divide the chicken among the tortillas. Drain the marinated vegetables and add to the tacos. Drizzle with the lime crema. Top with more scallions, cilantro and sesame seeds. Serve with lime wedges.