Checkout VAT extension allows you to validate VAT field in checkout shipping and billing address forms using VIES validation service. You can disallow order placement if VAT is not valid, set tax to zero when VAT number is valid.
The module works well with both Firecheckout and Standard onepage checkout modules.
Please note that VIES service can be offline sometimes and customer will be able to place order with invalid VAT number. Tax-free order will not work in this case.