I have just logged this with Enterprise who advise they have escalated the issue. I was advised that paid and 'gone free' apps that are no longer free should not trigger a 'requires credit card' request and they will investigate. my initial thought was that having bought several 100 licenses of a 'gone free' app that now had a price again was the problem apparently not.
Advised: VPP Managed Distribution should not trigger a requirement for a credit card on an enrolled Apple ID.
I have also told the Enterprise team, that there seems to be a few users on here with the same problem. He's not had any other calls. Therefore if you are being asked for credit cards when using VPP managed distribution - log the call.
The number to ring to log this is 0844 209 0611
Ask for Enterprise
The chap I spoke to was David Cronin.
I will post their response once they get back to me.