I was able to fix it myself in the end, using woocommerce_form_field_args filter :
add_filter( 'woocommerce_form_field_args', 'belair_vat_woocommerce_form_field_args', 10, 3 );
function belair_vat_woocommerce_form_field_args( $args, $key, $value ) {
if ( $key == 'billing_eu_vat_number' ) {
$user_id = get_current_user_id();
if($user_id){
$ws_vat_no = get_user_meta( $user_id, 'wwlc_cf_no_tva', true );
$eu_vat_no = get_user_meta( $user_id, 'billing_eu_vat_number', true );
$uservatno = $eu_vat_no ? $eu_vat_no : $ws_vat_no ;
}
if($uservatno){
$args['default'] = $uservatno;
}
}
return $args;
}