Better for someone else to build a Vulkan API on top of Metal, which is what has happened. It's not perfect, but it's the only thing that can work. The pressure on Apple should be for Metal to better support Vulkan by providing APIs it needs to work optimally.
Beyond that, Apple might want to contribute to the Vulkan-on-Metal implementation... though they're only going to do that if it makes strategic sense, which I don't see. For cross-platform, what makes more sense is to encourage games to use a higher-level engine that supports metal among its platforms, like Unity and Unreal.
Typically, if a game maker wants to make cross-platform a priority, they wouldn't target just Vulkan, they'd target a cross-platform framework. That would be true whether Vulkan was supported by Apple or not. And if they don't make cross-platform a priority, the chances of a mac port go down regardless.
So...
We're looking at the incremental gain of Apple providing first-party support for Vulkan vs the existing third-party support. Looks like a lot of work for Apple for little gain. Just doesn't seem worth it to me. Also, the Vulkan version would always be out-of-date since Apple would pin the supported version to an OS release, and would need to be conservative about it, since they aren't going to hold an OS release for Vulkan.
Really, Vulkan on macOS is much better done by the interested third parties, and the focus on Apple should be to get them to better support a Vulcan API on top of Metal.