Attached FileShould work fine. I used a hacksaw for mine then trued it up with a file and some sandpaper. Then rethreaded with the Carolina Shooter Supply thread kit.
12" on a Vepr is gonna cause all sorts of reliability problems. At the least you will want to polish up the hammer face, bolt carrier and bolt itself. Then make sure the gas block and gas ports are aligned correctly and none of the ports are getting blocked. After that is done you will have to enlarge the gas ports and more than likely (99.9%) have to add a 6th port as well. Even with all of that you will more than likely have to run 3 3/4 dram shells to get it to function reliably.
Mine will sometimes run 3 1/4 dram
shells but usually runs 95% with 3 3/4 dram. Thankfully the 3 3/4 stuff shoots pretty soft due to the gas system and can easily shoot 100+ and have little bruising on your shoulder. Well I will say that is with one of the massive GK 01, 02, or 03 brakes. I run mine with the GK03 brake. They make a huge difference in recoil. Shooting mine without a brake is brutal.
I would stick with 15" if you don't want to have to enlarge and add a gas port. You still might need to make sure the gas ports aren't blocked by the gas block, and still polish the internals.