Toothpick, I almost never mop or baste a butt, still, they come out moist and tender even for reheats.
That makes me believe that you're overcooking or reheating at too high a temperature.
To what internal temperature do you cook the butt? I normally stop cooking at 190* to 200* or when the bone turns easily. I pull, foil, and let rest for at least 1/2 hour.
To pack, I usually add a slight amount of water and rub then vac-pack, though Teddy Bear recommends adding a bit of Piedmont sauce, which sounds very good.
I usually place an unopened vac-pack of pulled pork in a pan of boiling water and heat until I can see the innermost part of the vac-pack boiling. If the center isn't hot enough, when you open the vac-pack, you can always place the pork back in the vacuum bag and boil a bit longer.
Keep an eye on the internal temperature of your butt, check for doneness by twisting the bone, and remove from the cooker when the bone turns easily in the meat.
Good luck!