I know that for fresh fish, or fish that was previously frozen without special storage, you should use it the same day. If I thawed too much, does the fact that it was vacuum-packed add to the time I can safely keep it? Does it depend on the type of fish?
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!
Apart from what Bob already mentioned, the enzymes of the fish will keep on working so the quality (independent of food safety) of the fish will go down rapidly over time.