Custom field required/not required in tax

Hi,

i'm struggling with a bit of a problem here, i have a customer that is selling to custumers inside belgium where they have to pay 21%tax and 0% tax outside belgium if the customer has a VAT number,

how can i make it required that when selecting that the customer has a VAT numer it needs to be filled in and when the customer selects no VAT the required field no longer becomes required?

anyone?