Did you check your spam folder to see if a receipt may have been miss-filtered? The game should still be listed under Your Games, though.
Under Your Account, does the address listed match that of the credit card you used (or Paypal)? Even with digital purchases the address can be used as a security check. I'm not sure if the Larian Vault does, but I had another site reject a pre-paid credit card because there was an extra space in the address (it declined the first order right away, though, and accepted the second just as fast once the addresses matched).
If nothing shows up in a few hours, email support@larian.com. You could email now, but it is the middle of the night in Belgium, so you might as well see if there was just a delay processing the order.