SourceCoast is a US based company. We don't use your VAT ID in any way. It's simply listed on the form as a courtesy for companies that need their VAT ID printed on the invoice we automatically generate. If you don't have a VAT, or don't care to have it on your invoice, you can skip that field.
I hope that helps answer your question, but if you need more assistance, just let me know.
Thanks,
Alex