zueth, I have done several briskets the way you propose. My last attempt, I smoked a 17 pound packer on the pellet pooper at 170 degrees for about 3 hours. I vacuum sealed it (double bagged and double sealed) and put it in the 10 gallon hot tub at 140 degrees for 60 hours. Then, dried it off, seared it on the gasser with help from a propane torch until it had a good sear. It turned out to be the best brisket I have done. It was melt-in-your-mouth tender, very juicy and moist, and had just the right amount of smokey brisket flavor. It takes a lot of time, but it really paid off and I will continue to enjoy great brisket done this way.
By the way, if you decide to SV a brisket, pork butt, or anything that is big and needs a long time in the bath, be sure to double bag it and vacuum seal both bags. I had a water breach on a rather large prime rib roast because my sealer strip had gotten grease on it and it didn't seal properly. The bag lost its vacuum and water got inside so the roast just cooked in water. It was still edible, but didn't have that great smokey taste it needed. Better safe than sorry.
There is a video on youtube by "Sous Vide Everything" that features this method and this is the method I follow.