Once it’s the day of departure and there’s a delay, award availability becomes irrelevant. You should be armed with a list of alternative routings to request from an AA agent in case there’s a delay. Don’t worry much about availability, since that can be fluid. Just have alternative routings out of SFO and ORD, for example the VN nonstop SFO-SGN (which operates three days a week according to ExpertFlyer). Preference for oneworld is reasonable, but have alternate routings on all airlines.

Whether AA is willing to put you on a non-oneworld airline will likely depend on the cause of the delay. They’ll be more willing if the cause is under their control (eg maintenance they can’t pretend is weather-related) than if it’s weather or air traffic control, I’d guess. If there’s a delay “outside their control”, I would expect to be in the hook for any ground costs (meals, accommodation) yourself.

All that said, though I haven’t done this particular connection, 90 minutes on a single ticket doesn’t seem very high-risk to me. Sure, delays happen and it’s very good to be prepared, but you should be able to absorb a fairly substantial delay. All the normal advice applies: you’re more flexible if you travel carry-on only, and have a few days of clothes in your carry-on so if you switch flights last minute and your bags don’t make it, you’re ok without them until they arrive.