Hi Blacknight,
Thanks for taking the time to share the details of your operational need with us.

It would be really helpful to hear some thoughts on how WHMCS admins envisage the ideal client and business operation experience in this situation. For example:
1. If a VAT number can't be validated with VIES, should the system allow the client to submit their order with any value in the VAT Number field?
2. If so, the Client Account could not be confirmed as tax exempt. Would you prefer if the system charged VAT or presumed tax exemption?
3. From a business operations perspective is it preferable for you to (i) handle the CS inquiry to manually validate a VAT number to mark accounts tax exempt, or (ii) handle the possibility that some orders may be submitted with invalid VAT numbers and not charged VAT.
4. If an order was submitted with a VAT Number which couldn't be validated by VIES, could the system notify admins in some way?
5. If so, what notification mechanism would be most useful (Email, Notification Rule, To-Do List, Support Ticket)?
6. Should subsequent changes to the Client Account's VAT Number via the Client Area be handled with separate logic, or the same way?

We appreciate your feedback in helping us design features which best meet the operational needs of our users.