Can I pay for a Vault once or every year?

Yes. Under How do you want to pay? you pick Pay once, for 1, 3 or 5 years (a longer term costs less a year), or Rolling yearly, where the Vault renews every year at the price of one year until you stop it. A Vault paid once ends at the end of its term unless you extend it. No VAT is charged: the company that sells Panoramr is not registered for VAT, so the price you see is the price you pay. If that changes, VAT will be added to the price of payments made after the change.

See Vault billing: pay once, or pay by the year.