Are long brines neccesary for sous vide?
Typically brines are for tenderizing and penetrating flavour into the meat. 

The use of the brine would be to break down the tough meat

Though a sous vide does tenderize mildly with the long cooking time you stated, you don't want to over cook it, hence the brine.

Nor would the sous vide alone add as much flavour as the 2 week brine.

The brine is more for a tenderization than to prevent drying out.