Hey @astro,
As promised, here is a temporary solution until the HivePress team decide whether to add this to core: Verification Expiry for HivePress 
It covers what you described, from the vendor’s side. An admin can set a verification period or a date of their own on each Vendor, or set a site-wide default, and the verified status is removed automatically when the date passes.
The vendor gets your two emails, one when they are verified and one when it expires, plus a reminder a set number of days beforehand, and all three are editable under HivePress > Emails like the standard ones.
There’s also an option to sync Listing verification badges with a Vendor’s verification badge, since HivePress keeps the two separate.
Cheers,
Chris 