I have had freezer burn with vacuum packed fish due to two causes:
1. The seal original was not good, usually caused by water or slime. Dry the fish, and wipe the mouth of the bag dry before sealing.
2. The bag punctures from harsh handling. Don't throw your frozen fish around, as the bags are fragile when froze.

No more freezer burn after taking care of these two items, and I have fish over one year in the freezer that still looks and tastes great.