First, stop using the vinegar as it can damage plant roots. Next be sure that the plant is being watered properly. Gardenias are often potted in mostly peat moss which can dry out and be hard to re-wet. Your plant may be too dry which often leads to leaf yellowing. Be sure the plants have good humidity (if they are house plants). Low humidity can cause flower buds to drop. Keep using the fertilizer for acid loving plants. It will help.
