Magento: Add Taxvat from customer to billingaddress -

i wondering how vat number being saved in billing-address customer. can't find in code , there no solution found online. can me out.

reason why i'm looking this: magento core 1.7 provides eu vat service instead of installing 3th party extensions. looks @ vat number saved in address instead of vat number customer. unfortunately it's not possible add vat number in address create in "my account" default.

big can out!

in invoice pdf use:

$taxvat = $order['customer_taxvat']; 

and echo need it.

in admin suppose can use {{order.customer_taxvat}}, haven't tested yet.