Loki Checkout Eu Vat
Within the LokiCheckout, we simply make use of the Magento core capabiliteit to validate VAT numbers. And it is made visually appealing if validation errors occur.
Whenever a customer enters a VAT number, the LokiCheckout is able to validate this number properly. First of all, a field prefix is populated with the current country value that the customer supplied (including a remapping when the VAT country is not equal to the Magento country ID, like with GR and EL). Next, the customer is able to enter a VAT ID that starts with that prefix. If the VAT number does not validate, it is not added to the quote.
Integration with Geissweb
The EU VAT Enhanced module of Geissweb is a popular solution to validate EU VAT numbers. While this third party module is not required for the validation of modules itself, it is handy to have anyway.
See LokiCheckout_GeisswebEuvat