Impeller already does what you're asking automatically and actually doesn't depend on Vulkan. On iOS and macOS, Impeller uses Metal, not Vulkan. On Android, it uses Vulkan if available, and otherwise falls back on OpenGL.

From the page you linked:

Impeller uses, but doesn't depend on, features available in modern APIs like Metal and Vulkan.

Flutter enables Impeller by default on Android. On devices that don't support Vulkan, Impeller will fallback to the the legacy OpenGL renderer. No action on your part is necessary for this fallback behavior.