Combine the red wine vinegar, olive oil, Italian seasoning, garlic, thyme and onion powder in a bowl.
Slice each chicken breast horizontally to make 4 thin cutlets. Put the chicken in a large zip-top bag or airtight container and pour the mixture over it.
Seal the bag or container and refrigerate for 3 to 5 hours, turning the chicken once or twice.
Preheat a grill or grill pan to medium-high heat. Remove the chicken from the marinade and discard the remaining marinade.
Grill the chicken for about 3 to 5 minutes per side, depending on thickness, until the thickest part reaches 165°F (74°C). Rest the chicken for 5 minutes before serving or slicing.