i added a 1/4 cup of vinegar the other night when making queso blanco with a gallon of whole milk. as with your situation it did not completely curd for about ten minutes. i then poured more in from the bottle periodically while stirring, within 30 seconds it had completely curddled. after draining the whey i mixed in garlic salt. not vinegar taste transferred over to the cheese, it does have a nice garlic hint.