Provided you've added your Apple account number and completed the enrollment process, the content manager should be able to sign into the VPP store with their managed Apple ID. VPP is still currently external to Apple School Manager, so clicking the 'buy apps & books' will open the VPP EDU store in a new tab.
From there it's the standard process of logging in, downloading the token, associating the token with Jamf Pro, and purchasing content (see "Integrating with VPP" in the Casper Suite Administrator's Guide).
