Grill peppers, without grill lid, over medium-high heat (350 to 400), turning often, 5 to 7 minutes or until peppers look blistered.
Place peppers in a heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel peppers; remove and discard seeds. Chop peppers.
Combine peppers, oil, vinegar, and garlic; cover and chill at least 8 hours.