Apple's prototype is basically WebMetal, and the question about how well it maps to Vulkan is still open. Like, surely, there are hoops you've got to solve (w.r.t memory types, pipeline barriers, etc), but it's not clear how much actual real-world performance is going to be lost on this emulation.

Same goes to the other direction (mapping from Obsidian/WebVulkan to Metal), with a caveat that Vulkan requires more state to be communicated, so it's easier to ignore some of the user input (if it doesn't make sense on Metal) than to figure out the missing bits.