This is a tough one to answer! When I google "apple cider vinegar toner," I get a lot of hits from bullshit websites lauding it for getting rid of toxins and being natural - the same sources that are going to tell you to use coconut oil as sunscreen or ingest essential oils. (Do not do either of those things, please.) Lots of people apparently think you should put apple cider vinegar on your face, but not many (any?) of them can provide an explanation of why, or at least one that isn't full of shit. "It's good for your pores/acne/balance/wrinkles/glow/dark spots/fish scales." Okay, but why?
Here is a post that talks about some of the potential dangers of using ACV on your face, including chemical burns (warning: graphic injury photos). ACV has some anti-bacterial properties, so maybe that could help with acne. But there is also good bacteria on your skin, so you don't want to annihilate it all. I'm guessing that back when people used harsh, alkaline soaps, an acidic toner like this could help to restore the pH balance of your skin. Using something overly basic (in pH terms) on your skin could also make it more prone to breeding bad bacteria, which the ACV might then kill. Maybe? But most cleansers now have a gentle pH, so that's not particularly necessary. And the pH is too high for ACV to be an effective exfoliant.
I'm not going to tell you not to use ACV on your face, so long as you are careful and dilute it well with water. I also can't recommend it, though, because I have found absolutely no evidence as to how it might be beneficial. When I google "apple cider vinegar toner science" most of the results include some variation of: "while no scientific research verifies these claims . . ." or, "I don't need scientific evidence to know it works, because . . ." Uh huh.
(ACV might be good for your hair, though. Or maybe not. That's a whole other thing, anyway. Also, you can use it to make a safe mask if you mix it with clay with a higher pH.)
Anyone have an better information to share on the topic? I mean something other than anecdotal stuff. My usual sources are coming up dry.