I'm confident we can expose something like Vulkan on top of Metal without too much friction.
It's the "like" bit that worries me, a bit. I'm sure Epic and Unity3D would find it rather convenient if they could plug their Vulkan backends more or less straight into the web (via web-assembly no doubt). It's also the case that all the documentation, books, references, tooling, middlewares, etc. that do start to exist for Vulkan, would help.
If you can par down on Vulkan but keep it vulkan compatible (insofar as that doesn't conflict with the webby-ness of the web), fine, call it Vulkan ES or something. The trouble starts when Vulkan ES doesn't conform to the principles, signatures and behaviors that Vulkan establishes, just as that's everytime incredibly troublesome when that happens in GL land (floating point textures anybody?).