FWIW: I believe you can get some of this info by re-creating the itinerary on ITA. At least, when I have asked ITA to show me options for a specific CO B-fare, and it displays some UA-operated flights, it actually notes that those flights book into the V-booking-code (screen-shot below):


It's even more confusing since in this itinerary, the LAX-EWR flight has a CO B-fare-basis, a UA-flight-number, and operated-by-CO, but still seems to get a V-booking-code.

However, sometimes it shows UA flights on the same fare that show a B-booking-code, so I don't know what is used to determine the mapping. It may be fare/route-specific ???