If you can get about a spoonful of the drippings (or if you make au jus) in each bag, that seems to work great. It's not too much where the FoodSaver sucks it out and it's just enough to get some moisture in the bag when you're reheating. Make sure you get it in the bottom and don't let it run down the sides or else it will be a pain for the FoodSaver.
Reheat is easy... bring water to the start of a boil, then turn to simmer. Drop the bag in for 20 min and done (this is for FoodSaver bags, not sure of other brands). I've frozen/reheated brisket, pork butt, and half chickens. All end up coming out great from the freezer.