Cook the rotini pasta according to package instructions. Drain and rinse under cold water to cool.
Preheat the grill to medium-high heat. In a small bowl, mix olive oil, garlic powder, Italian seasoning, salt, and pepper. Brush the mixture onto the chicken breasts.
Grill the chicken for about 6-7 minutes on each side, or until fully cooked and internal temperature reaches 165°F. Remove from the grill and let it rest for 5 minutes before slicing.
In a large bowl, combine the cooled pasta, cherry tomatoes, cucumber, red onion, feta cheese, and basil.
In a separate bowl, whisk together balsamic vinegar, olive oil, Dijon mustard, honey, salt, and pepper until well combined.
Pour the vinaigrette over the pasta salad and toss gently to combine. Add the sliced grilled chicken on top.
Serve immediately or chill in the refrigerator for 30 minutes to let the flavors meld.