This has been discussed quite a bit --
https://www.google.com/search?q=vinegar+in+soap+site:soapmakingforum.com&ie=utf-8&oe=utf-8
A modest amount of any acidic ingredient will react with some of the lye. The net effect of this reaction will increase the superfat, but it will NOT reduce the pH of the finished soap. Citric acid in citrus juice ends up as sodium citrate. Lactic acid in yogurt makes sodium lactate. Acetic acid in vinegar makes sodium acetate. Etc.
As in all things, moderation is a good idea. If you add enough acid as was done in the Soaping 101 "make soap with lemons" video, there will not be enough lye left over to actually make soap. You'll end up with an ugly mixture of soap, fatty acid, unreacted fat, etc. In that case, yes, the pH of the "soap" will be reduced ... but you really don't have soap at that point...
At a 5% acetic solution, 30 g of commercial vinegar will neutralize 1 g of NaOH or 1.4 g of KOH. If I use commercial vinegar for all of the "water" in my favorite soap recipe and don't add any extra lye to compensate for the added acid, the vinegar will eat up enough of my soaping lye to increase the superfat from my usual 3% to a substantial 10%.
ETA: I goofed! Aprice is asking about apple cider/juice NOT vinegar. Duh. I'll leave this info about vinegar in case someone can use it, but please understand my post here applies to vinegar, not apple juice.