Here's what I've done several times:
1) Go to a thrift store and look for a leather or vinyl coat or long jacket just a bit larger than you need, in the color you want....make sure it’s smooth, or at least has 1 seam down the middle only….
2) Fold the collar inside, and try the coat/jacket on BACKWARDS, and snap/zip/button it up...
3) If it fits relatively well, and it's cheap, then you can buy it and take it home....
4) Remove the collar (use seam ripper or cut it off—if you cut it off, get close to the existing seam)...
5) Remove the sleeves (ripper or cut them off)...OR...you can cut a semi-circle pattern and leave the tops of the sleeves for caps (Jango-style)...OR...you can use the spare leather of the pockets for the caps later....leave a little for hem allowance.
6) I you use a long jacket, measure yourself from the top of your shoulder, down to where you want the bottom of the vest to stop, mark it, and cut and remove the extra material at the bottom....leave some hem allowance.
7) Cut the neck opening into a semi-circle (the back of the collar) so it feels right, and doesn’t choke you….leave some hem allowance.
8 ) On the reverse side (now the back of the vest – really the front placket), use Velcro, snaps, zipper, etc. for closure fasteners….you may have to move the buttons or add snaps to make it close better…
9) If it’s a bit baggy under the arms (and it will be, as you are now wearing it backwards, and it wasn’t cut for that), you can use safety pins or sew a folded dart under the armpits and down the sides to take up the extra cloth, and make it a custom fit…. if you place the closures/buttons right, you can take some of the "slop" out then.
10) Seam allowances: If it’s a leather vest, and you don’t have a heavy-duty sewing machine (and don’t try to sew leather with a standard machine, or you’ll strip the nylon dog gears), you can either sew it by hand, or use hot glue…just fold them hem allowances over and use a spot of glue.
11) Add the pockets or use some left-over fabric for sleeve caps (after making a semi-circular pattern for cutting), using hot glue or hand-sewing (if leather) or machine (if vinyl) to attach at the top shoulder seams….
12) That’s it! Now you can attach your armor plates via Velcro or the stud/grommet system…
Rob