Just a further note that I also needed to edit the WA_Protx_VSPFormHash() function definition to add the $tBillingState and $tShippingState to the variables list:
function WA_Protx_VSPFormHash($theCart, $tPassword, $tVendorTxCode, $tAmount, $tCurrency, $tDescription, $tSuccessURL, $tFailureURL, $tCustomerEmail, $tVendorEmail, $tDeliverySurname, $tDeliveryFirstnames, $tDeliveryAddress1, $tDeliveryPostCode, $tDeliveryCity, $tDeliveryCountry, $tBillingSurname, $tBillingFirstnames, $tBillingAddress1, $tBillingPostCode, $tBillingCity, $tBillingCountry, $tContactNumber, $tContactFax, $tAllowGiftAid, $tApplyAVSCV2, $tApply3DSecure, $tEMailMessage, $tBillingState, $tShippingState) {