Found the way:

To drill to the selector I need I have to create 2 "entity has field" conditions.

1. entity "site:current-cart-order" has field "commerce_costumer_billing"

2. entity "site:current-cart-order:commerce-customer-billing" has field "vat_number"

then I can select the data selector

Found the way:

To drill to the selector I have to create 2 "entity has field" conditions.

1. entity "site:current-cart-order" has field "commerce_costumer_billing"

2. entity "site:current-cart-order:commerce-customer-billing" has field "vat_number"

then I can select the data selector