Mix the balsamic vinegar, olive oil, and black pepper in a sealable container the night before, if possible. Poke the tomatoes a few times with a fork or sharp knife. Add them to the container. After they’ve soaked for about 30 minutes at room temperature, thread them onto grilling skewers (save the marinade).
Heat a grill to medium-high. Grill the tomato skewers for 1 minute on each side. Let cool, then slice the tomatoes in half. Add back to the marinade with the remaining ingredients and toss to combine. Refrigerate for at least 2 hours before serving.