Nothing to worry at your end. If you received invoice without vat and it is recommended to take an undertaking from the supplier that they are not registered for vat and not required to be, this is part of best practices and not as required by low. if your dealing with them in subsequent 12 months is less than Aed 375,000, you may not be held liable against your supplier acts. But recommended to stop the dealing, if they are already dealing with you in more than the said limits.