For iOS 6 and previous, when you issue an app purchased from VPP to a device - it becomes permanently tied to the Apple ID just like gifting an app burns a redemption code once it gets redeemed. Apple has publicly announced that VPP codes will be able to be reclaimed and reused indefinitely by the "purchaser" so this will likely be a pain of the past once iOS 7 ships.
Here is the marketing pitch from http://www.apple.com/ios/ios7/business/ under the App Store license management heading:
The App Store Volume Purchase Program now offers businesses the
ability to assign apps to their users while keeping full ownership and
control over app licenses. Companies purchase app licenses through the
VPP website, and can use their MDM solution to assign apps to
employees over the air. Employees can enroll with their personal Apple
IDs without providing it to their company, and apps are placed in
their purchase history for self-service download, or are installed
automatically via MDM. Apps can be revoked at any time and reassigned
to other employees. In addition to iOS apps, VPP now supports the
purchase of Mac apps and even books, so users can be provided all the
tools they need to get the job done.
Documenting how things are in the Summer of 2013 and earlier, here are the options we have.
There are some circumstances where you could use the Apple Configurator to manage VPP in a non-burned state for certain managed device configurations (think of a lab cart with 30 or more iOS devices, all configured the same for check in and check out like a library book where many people will use one app) but you need to safeguard the Mac since all those licenses are tied to that one computer so they are "burned" in a different manner than gifting / traditional VPP burns them.
You can have a policy where VPP are only issues to a company Apple ID, so you can rename that ID or use generic accounts and recover a "slot" for a new person when people change roles.