The main "point" of sou vide is you set the water at the exact temperature you want the meat to cook to and it will evenly cook it throughout because it's in the circulating water. An oven, you need to set the temperature higher so you get it more done on the outside and less done on the inside which often means you end out under cooking or over cooking. The vaccum sealed bag just keeps the water off the meat. I don't think it would help to put it in a bag in the oven.
Google SouVide. The whole set up can be real cheap. You only need the machine. You can use any old giant pot and you don't even need to vacuum seal, you can use zip lock bags and clip them to the side if you're careful.
If you do try the oven and bag method, put it into an oven safe pot full of water. The water will keep the temp from fluctuating as the heating element comes on and off
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