Combine tomatoes, 1/2 onion, 2 tbsp. oil, 1 clove garlic, vinegar, salt and black pepper in a medium bowl; let salsa stand 15 minutes.
Remove stems from poblano peppers by cutting each pepper about 1/2 inch from stem; remove seeds. Grill peppers over medium-hot coals until skins are charred on all sides. Place peppers in a large resealable plastic food storage bag; seal. Let stand 5 minutes; remove skin. Grill corn over medium-hot coals 6-10 minutes or until tender, turning every minute; cut kernels from cob.
Combine corn, cheeses, minced cilantro, remaining 1/2 onion and 1 clove garlic in a medium bowl; mix well. Carefully fill each pepper with cheese mixture - making cut in side of pepper, if necessary. Secure opening with wooden picks that have been previously soaked in hot water for 15 minutes to prevent burning). Brush peppers with remaining 1 tbsp. oil; grill over medium coals 1 minute per side until cheese melts. Serve with salsa. Garnish with cilantro sprigs.