How long can I keep thawed vacuum-packed fish in the fridge?
There's always the delight of botulism, which thrives in the anaerobic environment provided by vacuum packing.  Listeria and vibrio bacteria can grow in an anaerobic environment, and are potential food safety risks present in fish.  

With this in mind, I would not expect vacuum packing to extend the life of a fish much.  It will reduce freezer burn though!