If you have several tax numbers and don't know, which is the VAT-ID, check this European database:
http://ec.europa.eu/taxation_customs/vies/vieshome.do?selectedLanguage=EN
The VAT-Id starts with two letters representing your country (e.g. DE for Germany).
Then some digits are following.
Example: The VAT-Id of the top500 company is DE163709325
More about the VAT-Id or the VIES (VAT Information Exchange System) can be found here:
https://taxation-customs.ec.europa.eu/taxation/vat/traders/vat_number/index_en.htm