https://ec.europa.eu/taxation_customs/vies/technicalInformation.html
This dart lib is basically calling web service provided by VIES (at https://ec.europa.eu/taxation_customs/vies/checkVatService.wsdl) for VAT number validation.
If VAT is valid this lib return business entity infos.
With Dart:
dart pub add vies
With Flutter:
flutter pub add vies
Enjoy.