Preheat the oven to 425°F (220°C). Cut four 18x12-inch (45x30 cm) pieces of heavy-duty aluminum foil and lay them flat on your counter.
Pat the chicken thighs and drumsticks dry with paper towels. Season the chicken evenly on all sides with the kosher salt, garlic powder, and smoked paprika.
Place 3/4 cup (85 g) of the frozen mixed vegetables in the center of each piece of foil.
Arrange one seasoned chicken thigh and one seasoned drumstick on top of the vegetables in each packet. Brush the chicken pieces with half of the barbecue sauce, using about 1 tablespoon per packet.
Bring the long sides of the foil together over the chicken and fold them down twice to create a seal, leaving a little space for heat to circulate. Fold the short ends in twice to close the packets completely.
Transfer the sealed foil packets to a large rimmed baking sheet. Bake for 50 to 55 minutes, or until the chicken reaches a safe internal temperature of 165°F (74°C) on a meat thermometer.
Remove the baking sheet from the oven. Carefully open the top of each foil packet, watching out for hot steam.
Brush the chicken with the remaining barbecue sauce. Leave the foil open and return the baking sheet to the oven for 5 minutes, until the sauce thickens and forms a glaze.