The vent cap is just that, a vent for the diff housing . It has nothing to do with the brake fluid / lines other than a holder for the the lines.
I would check and see whats dripping down over the housing before repairing anything. Just rotating the vent cap sometimes is enough to make sure its venting properly.
If its brake fluid , you may have a bad flare on the line, fitting could be cross-threaded or the brass block could have a hair line crack in it.
Once you determine the type of liquid on the housing then start to repair .