You can definitely soak garlic in apple cider vinegar, but the process is called pickling rather than fermentation. Pickling involves placing herbs or vegetables in an acidic brine (vinegar) in order to preserve them, while fermentation involves letting good bacteria produce lactic acid, vitamins and enzymes over time. The final products of both are acidic, but they're different. Both have their place - my beloved fire cider recipe is pickled and I love the stuff. In fact, there's a batch brewing in my kitchen right now.