Sous-Vide cook out juices
The issue with accomplishing what you want to in the bag is the low temperature typically used in sous vide cooking.  These low temperatures prevent the proteins from coagulating.  This can only happen once you dump the juices into a pan to finish a sauce at higher temperatures. Then, you will probably notice that you get globs of coagulated proteins, which are less than appetizing.  So, you will want to pour the juices into a pot, bring to a simmer, and skim the proteins (or strain them out).  Now you can use the broth as a stock to create a sauce.  Remember though, you will have to build flavor, as these juices are generally not that flavorful.  In your case, you will need to bring to a boil, skim or strain, then reduce significantly to create a glaze.