The Spacex crew Dragon uses the Falcon 9 for its orbital launches, and is not set to ever use a different launcher.
However, an uncrewed Dragon will probably launch on the Falcon Heavy, as is planned with Dragon XL for the Artemis program.
The Boeing Starliner is designed to be compatible with four launch vehicles: Atlas V, Delta IV, Falcon 9, and Vulcan, even though it will likely only ever use Atlas V and eventually Vulcan.
This answer strongly suspects that Crew Dragon is rated for at least Atlas. Is this the case?
Did Spacex design Dragon to be rated for other launch vehicles, like Atlas V?
What are the incentives for them to do so? (Any directives from the US government?)
Could ULA do it the other way and design Vulcan to be compatible with Dragon?