Or perhaps Geraldo could do a 2 hour special leading up to the revelation of the "secret".
Apologies to the original poster for the digressions, but just about anything wet will make more rub stick to the meat. Not sure I see the superior benefit of apple cider vinegar, though. For pork, most use yellow mustard; Worcestershire for beef. Even just salting and waiting a bit before applying rub will pull moisture to the meat surface and allow more to adhere. And, I think, no matter what you do, there's not going to be all that much rub "getting into" the meat. Once the cook is under way, fat renders, juices flow, sugars carmelize, and the combination of all that leads to what we like about the texture and flavor of the bark of BBQ meats.