Air carrier pilot reported confusion with published guidance reference to execute approach with an FMC database limitation and ground equipment outage.

Narrative

SBA was landing Runway 25 utilizing the VOR or GPS Runway 25 approach. The FMC data base on Aircraft X only had VOR 25 available (no GPS 25). The ground based navaid for this is GVO VOR which was NOTAMed out of service. This led to a considerable amount of confusion and a couple of turns in holding while trying to sort out the legality of flying the VOR approach with the VOR out of service. I eventually determined that it could be flown using the 'RNAV Substitute Approach' provisions of the FOM; but it's still a bit confusing as the FOM page references the AOM for authorization; and the AOM references the same FOM page for authorization. Somewhat circular authorization it seems. This is the first time I've encountered this. Are all the Aircraft X FMC data bases like this (only offer VOR as the named approach when it's a VOR or GPS approach in FD Pro)?[Caused by] rare event: ground based navaid for approach out of service; confusing approach naming criteria in FMC data base; difficulty determining legality/authorization in FOM and AOM; lack of reminders in flight plan/release or Jepp/FD Pro company (10-7) pages of the RNAV Substitute Approach provisions when navaids are unavailable.[I suggest] a note in the 10-7 pages for any airport utilizing VOR or GPS approach charts that RNAV Substitute Approach provisions are available if the FMC data base doesn't specify or list a GPS approach for that runway in the event the ground based navaid is out of service.