The main difference with PayPal Payments standard is that Payments standard is not a real time gateway.
What this means is that when the user is returned to the your site to the success page, it only means that the payment method was accepted by the paypal server. at this point the transaction has not been sent to the merchant bank.
PayPal uses a system called IPN (Instant Payment Notification), this send a post to a page you set up on your site when the transaction goes through the merchant bank with the success or failure result. for more details on IPN, see the following page:
you could configure the IPN page to send an email using Universal Email with the cart contents on payments success
with Sage Pay VSP form method, this is a real time gateway, so you could add universal Email to the checkout success page to send the cart contents.