After you set up Google Vacation Rentals, a property URL that uses the source ID may return a 404. There are two common causes.
Existing properties have not been re-synced. When you enable Source Id as the secondary slug setting, it only takes effect for properties imported after the change. Any property synced before you updated the setting still returns a 404 at its source-ID URL until you run a sync on that property. To fix this, sync each property that was imported before the change.
The URL path does not match your site’s property URL format. Sites use different base paths for property pages. Some use /unit/, while others use /property/ or another base. The URL that Google receives from the feed must exactly match what your site serves, so if the base is wrong, every listing URL returns a 404. Check that the base path in the URLs Google receives matches the property path your site uses.
Contact support if syncing the property does not resolve the 404.