Deployment of free enterprise apps in VPP
From https://support.apple.com/en-us/HT202995 :


  Free apps are available as redeemable codes solely for custom B2B apps.


That means you can distribute Free Apps with codes but only if the developer assigned your VPP Apple ID for a custom version of the App (might be the same as the generally available free version, but you need to contact the developer and request they publish it to you).