Freeze your stock in a tightly sealed container (the vacuum sealer will do a fine job but isn't the only option) and don't worry about the 3 month time limit.
Freezer burn is nothing more than dehydration. Have you ever noticed how ice cubes that have been in the freezer for a while shrink in size? That's freezer burn. The freezer cycle draws moisture from a frozen item. As shrimp stock is frozen flavored water, freezer burn will just evaporate off some of the water. It won't effect the flavor.
I freeze chicken stockfor longer than 3 months all the time and have never noticed an off taste.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan