I doubt NASA wants to put 4 astronauts on the next Starliner flight. So if NASA declares this crewed flight test a failure and requires a redo (and possibly even reverting back to a third out of one planned uncrewed flight test), Boeing is still on the hook for their operational 6 crewed flights.
Here's the problem: Starliner flies in the Atlas 5 rocket. Which is officially deprecated and all of the vehicles that will ever built have been booked. Which would mean that Boeing has to nicely ask Project Kepler for one (or more) or their remaining Atlas 5 slots. All of this also pushes back the final flight of the Atlas 5. Starliner already has 5 years where it's the only mission in that rocket, requiring hardware and operational knowledge to be on retainer just for Starliner. At least the pad that launches Starliner can also do Vulcan launches, so they won't be hogging a launch pad just for this problematic program.
If they manage to human rate Vulcan, I don't think it would be technically hard to launch Starliner on it. Just looking at Cygnus which used three different rockets from three different companies.
Here's the problem: Starliner flies in the Atlas 5 rocket. Which is officially deprecated and all of the vehicles that will ever built have been booked. Which would mean that Boeing has to nicely ask Project Kepler for one (or more) or their remaining Atlas 5 slots. All of this also pushes back the final flight of the Atlas 5. Starliner already has 5 years where it's the only mission in that rocket, requiring hardware and operational knowledge to be on retainer just for Starliner. At least the pad that launches Starliner can also do Vulcan launches, so they won't be hogging a launch pad just for this problematic program.