The Paid Subscriptions feature of vBulletin allows you to set up a subscription for your users. Various aspects of this can be adjusted to suit your needs such as the amount that is required, the length that the subscription lasts and if the user will be moved to a different usergroup.
Once the subscription is expired, it should revert them back to their original group.
https://www.vbulletin.com/docs/html/...scriptions_add