Our bodies turn glucose and ketones into ATP which is the basic 'petrol' that we burn for energy. ATP is akin to acetic acid, so it will need less of our energy to strip it down to ATP, and thus is more easily digested, It is not the apples that is important, but the vinegar. Whatever flavour you find palatable. I use balsamic vinegar and olive oil vinaigrette on salady thingies. I do not drink neat vinegar as a matter of choice, and also because the first stage of digestion is very acidic anyway, and I don't want to create ulcers by going too acidic.
Something I do not understand is why some diet plans that purport to reduce the acid content of our normal foods suggest a large daily intake of ACV. Seems counterintuitive to use an acid to quell an acid.