A sweat shirt, new ($5-$10) or used (raid the closet)
24” jacket zipper (+/- $3, this kind releases at the bottom so you totally open your vest)
Lay out the sweat shirt on a table for cutting. Cut off both sleeves. Set aside, these will provide the material for the pockets. Did I forget to tell you that this vest would have pockets? Okay, it will have pockets.
(At this point I will apologize that this sweat shirt is so dark. I didn't realize how hard it would be to photograph. I will remember this and consider it a lesson learned.)