Is there anything wrong with preparing a butt the night before cooking by injecting and applying a rub and vacuum sealing it? I know the salt in the rub will initally pull moisture out but it should equalize sitting overnight.
It's up to you. If you're doing it for convenience sake there's nothing wrong with it rubbing it and letting it sit overnight at all. Just be aware of a couple things:
If the rub or injection is high in salt it may have a slight curing effect on the meat making it taste a little hammy.
Also, for the rub, only water soluble things like the salt & sugar will be able to penetrate unless your letting it sit for days. The spices themselves don't penetrate, because they're not water soluble. While cold, it takes days for the flavors to leach out of the spices and into the meat. When they heat up during cooking, they release their essential oils which can then be absorbed into the meat.
As far as vac packing, I agree with Rick; it's overkill (unless, of course, you plan to let it sit for days after applying rub).
I agree with Tent... Number one I dont inject. I have never liked the taste and texture when i tried it. Number two I rub and let it sit in the fridge for no more than six hours. Number three I never wrap at the stall unless I absolutely have to. If you dont have the time to cook right dont. But dont forget I am a bark man. Good luck dude!
I agree on the no inject. I tend to dislike the results. Brining however, which is basically the same thing, I have had mixed results: some good some so-so. I think that I prefer the heavy bark pieces mixed with some tender internal meat without bark. I may try cutting the butt into pieces to increase the bark ratio and decreasing the overall cook time. Also think that I might discard the foil step and just leave the bark to develop without foiling and just do some late mopping, combined with a smaller piece of pork so that it cooks faster might end up with ideal bark thickness and ratio of bark to non-bark pieces of pulled final product.