It wont harm the plants, however vinegar amounts to organic material in the res, which can potentially invite root disease. This is especially true if the vinegar is old and has had a chance to turn to mother of vinegar. Vinegar also breaks down fast so your PH will rise again sooner than professional adjuster.
I once knew a guy who was determined to save money on PH down. He said, I know a secret a grower told me years ago, you can use vinegar. (wow big secret) He quickly found that his PH would be high again just hours after he added vinegar. To combat this he set up a constant drip system. He eventually found the right drip rate to keep the PH at 5.5, and then he found that he was getting deficiencies because PH needs to range from 5.5ish to 6.3ish. Constantly keeping it at 5.5 was locking out some nutes. In the end he had an elaborate drip system that was counter productive and costs way more money and energy than if he would have just gotten a bottle of PH down.
Bottom line is, GH PH down works much better and doesn't cost that much more.