No, sales tax is not due on gold, silver, platinum or palladium purchases.
BullionVault gold, silver, platinum and palladium are all VAT (sales tax) free - for as long as they are held in accredited vaults. Gold generally remains exempt on withdrawal (though this may depend on the jurisdiction in which you live), but 20% VAT will apply if you choose to withdraw your silver, platinum or palladium bullion.