In a large bowl, combine the EVOO, vinegar, garlic and rosemary. Add the pork and toss. Refrigerate for at least 30 minutes or up to 4 hours.
Preheat a grill to medium-high. Season the pork with 3/4 tsp. each salt and pepper. Thread onto four 10-inch skewers, alternating with the onion. Lightly oil the grill grate. Grill the kebabs, turning once or twice, until the pork is firm to the touch and the onion is crisp-tender, about 10 minutes.