Get in line for the decade long wait for Apple to support subscriptions and in-app purchases via VPP.
It cannot be done.
VPP is designed and currently implemented for the one-time purchase of apps. There is no mechanism to allow for subscriptions or in-app purchases. And the current limitations do not allow managed Apple IDs to participate in subscriptions or in-app purchases.
Your only option is to use a personal Apple ID and associate it to a payment source like a credit card. This removes control of the purchase from the organization and may violate your deployment limitations of allowing personal Apple IDs to sign into managed enterprise devices.
I encourage you to reach out to ABM support (Contact Apple for support with Apple Business Manager, Apple School Manager, Apple Business Essentials, and Pro Apps - Apple Support
And to submit feedback to Apple. As someone who supports education, SMB, and enterprise, this is a request we've been asking for since forever. We have submitted feedback and feature requests with each iteration of ABM/ASM.