If you're injecting any vinegar or some other tenderizing agent it probably wouldn't hurt to do it the night before but hold off on the rub until later as the injection will probably want to seep out and will wash it off. I do however agree with spyre that injection isn't really necessary, there's plenty of flavor in both pork shoulder and brisket due to the fat content that will render out and baste the muscle internally. Apply a good liberal coating of your favorite rub, cook them at the right temperature for the right amount of time and they will be tender, juicy and flavorful.
Never Trust A Skinny Cook!!!