I looked around and haven't seen this posted before—sorry if it turns out I'm duplicating old posts. Here's to hoping this helps someone else out as much as it's helped me in the last week!
In a few of our school districts we're still using user-based VPP with generic users (can't wait for next year's reset for Device-VPP). They aren't tied to any JSS-based account or an LDAP server—only entered in the device's user record with an API script. That causes a problem if you're going to use the trick of going to yourjssURL/vpp to accept the VPP invitation as there's no password for those users.
You can, however, copy your device's UDID (Mobile Device record > Hardware > UDID) and use it to construct the same URL you use for the mobile device Self Service web clip: yourjssurl/mdss/?udid=(pasteUDIDhere)
If you have your VPP invitations setup to appear in self service, you can click on the "Accept" button, which kicks you over to the Mac App Store (helps to be signed out before starting). You can then sign in with that generic user's AppleID and accept the invitation.
