A quick Google search indicates that O is a AF business class fare so VS seems correct on that. I wish I could offer you some sage advice but I got nothing. This is one of the issues with partner bookings as each airline will point fingers at the other. Perhaps you can see if VS will accommodate you on their flights though that will require a connection in LHR which has its own issues especially now. Honestly I'd probably try to get a straight answer on AF on your return leg. You might also want to head over to the Saudia website (https://www.saudia.com/) and go to manage my booking and enter your e-ticket number from VS and see what you see in the ticket (it should work even though you are not flying Saudia). It will at least allow you confirm your fare code.

Edit: And it looks like AF is correct that O is economy on VS: https://v-flyer.com/toolbox_fare_codes.php . I would assume the fare code that applies here would be the operating carrier not the ticketing carrier but I don't know enough to be sure.