The Boeing Starliner is designed to be compatible with four launch vehicles: Atlas V, Delta IV, Falcon 9, and Vulcan.
The test flight was launched on an Atlas V, so it can be assumed that more launches will be using it.
What about the others?
Delta IV doesn't seem human rated yet, but they did propose to make it human-rated at some point. But would they bother using it when the Atlas V is already shown to work properly for Starliner?
Vulcan is intended to undergo human-rating certification to launch the Starliner, but will that include launches to the ISS?
Falcon 9 isn't associated with Boeing or ULA. Would Spacex even let a competitor fly on their launcher? Are there any plans for this?