I experienced the same leaf drop issue as well here. My winters are mild and temperatures zig zag so that contributed a little. I tried to remember not to fertilize about three months before my average date of first frost (the first week in November for Lynchburg).
Frost can damage some of the foliage and you got close to/below freezing 33F on April 1-3 when overnight temps dipped from 29 to 36 on those three days. I would now wait for new foliage, mulch and continue your watering program.
I would not try to use the diluted vinegar + water solution on gardenias' tiny roots. Some people use vinegar to acidify the soil but I find it causes problems with plants like azaleas, hydrangeas and camellias. These plants all have shallow, tiny, fibrous roots, similar to gardenias but gardenia roots do grow down a bit more but are still shallow (top 6" or thereabouts). Vinegar is used in organic programs as a weed killer so you could be injuring some of the gardenias' roots as the root system is still small. If your soil is not already acidic, I would amend the soil with either greensand or with garden sulfur.