Welcome to the "exciting" world of iOS VPP licensing. If you log into the VPP site (https://volume.itunes.apple.com/nz/store) and click on your email address in the top right, then click on Purchase History, you'll be able to download your spreadsheet of codes.
The two Apple ID's for VPP are thus: A manager and an Administrator. I could have this around the wrong way but essentially an Administrator can't do anything except create and assign managers, who can actually go out and buy things. Weird I know, but set and forget.
Redeemable Codes:
A one-way gift of an app to an Apple ID. Essentially fancy iTunes vouchers. If you're using Apple Configurator then you can pull licenses back off iPads and swap them around but if you lose your Configurator machine or the iPad goes funny the license is lost. Back up the Configurator!
Purchase app on behalf of a user and assign their Apple ID a license. Can revoke/reassign licenses at will. Everything is stored by Apple so licenses are never lost (apparently). Preferred method for OTA/non-Configurator deployments. Needs an MDM product to manage and unique Apple ID's per device to work as designed. Technically possible to use one Apple ID for a bunch of devices but getting into fuzzy ground with Apple T&C's.
Hopefully that clarifies things a bit.