Hi Ray. I've checked and the Encryption Password in Protx_VSPForm.php is correct for the SagePay test server that I'm using while updating the integration.
Previouly when it wasn't correct, I couldn't connect to the SagePay account when going to checkout - there was an encryption error at that point. So I think the password must be correct or I wouldn't be getting to the checkout success page back on my website. Could I send you the Protx_VSPForm.php file and the checkout_success.php page that is trying and failing to connect with it to update the order and mark as complete on my database?