The idea of 'more is better' is a dangerous one when it comes to your garden. Vinegar is what you use to pour on weeds when you really, really don't want them to come back. Excellent drainage won't save you if you miscalculate. The above recipe you have been given would be a bit strong for most soils but in your case, it sounds about right. Increase the vinegar at your peril - or the poor plant's peril. It wouldn't hurt you to discuss this matter with a local nursery; that doesn't have to cost you anything.