This problem cannot be solved, not with sous vide and not with something else.
Meat is made of cells, whose walls are made mostly of proteins. These proteins change their structure when heated, and that's what turns raw meat into cooked meat. When they change their structure (denature), you get tears in the cell walls.
When you have a piece of meat which is poor in collagen, say a steak, you only heat it enough to denature only the myosin, and most other proteins stay intact. Some of the juices flow out, but most of them can be held back by the still existing cell walls.
If you were to do that to meat rich in collagen, you would end up with impossibly tough meat. So you expose that to enough heat for long enough time that all the tough collagen changes thoroughly and turns to smooth, lubricating gelatin. Braising is one of the methods to do that. When this has happened, all the other proteins are far, far gone, and all the liquid from the cells has flowed out through the now-shredded cell membranes. If you are braising, it flows into the braising liquid, if you are doing sous vide, you will find it in the pouch. This type of cooking is incompatible with the juices staying in.
If you are missing flavor in braised meat, you might be braising the wrong type of meat. Some milder flavored meats like chicken and animals raised on mass production farms (little movement, no variation in food, no fat, slaughtered young) are simply not gamey enough. If you braise mutton, or a hog, or wild fowl, with some fat marbling too, you will certainly have flavor in the meat itself. Not because of juices, but because the meat is aromatic. Even worse, if you are braising meat parts from mild tasting animals which are low in collagen, you will not only lose the juices, you won't have the gelatin either, and you will end up with dried lumps of tough, tasteless matter.