It’s being actively worked on, so a release with Vulkan support shouldn’t be far off, and perhaps a lot sooner than trying to add Vulkan support any other way to Cocos2d-x. In this case, if Cocos2d-x was to use bgfx, then there would be no code changes required when Vulkan support is enabled.
The main concern right now is Cocos2d-x supporting Metal, and since bgfx has that support, it is a solution to the concerns raised, which is the reason this whole conversation started regarding bgfx. 