I just did this on a brisket my sister wanted to take home with her for her son. I cooked the brisket the whole way. Then I let it rest in a hot box for 3-4 hours and then sliced it. I saved the soppin's from inside the foil, defatted them and poured them into a vacuum bag along with the slices. When you go to seal the bag, use the manual seal process. Start the vacuum process and as soon as you see the liquid being drawn up towards the top of the bag, hit the manual seal button. This gets most of the air out and keeps the liquid in. It takes a little practice to do this correctly. And then I froze it.
I heard that they defrosted and reheated it and it came out great. Thanks to Tom's brisket recipe and expert brisket guidance!