Every time we have had devices prompting for an apple id, either the
assign VPP content was not checked in the app or all of the licenses
were used up. Resolving those two issues had made the prompting for the
apple id go way.
We are using device based assignment as well and are seeing this issue.
It is completely random. Upgrading to iOS 9.3.4 fixed it for some of our
devices, but we have others that are still not installing due to the
iTunes store ID could not be validat...