(In any case, is it yet certain that Apple won't support Vulkan on OS X? It wouldn't have been ready for El Capitan's launch this Autumn regardless. Maybe Apple will try to deprecate it (in the non-technical sense) as a compatibility option but support it anyway?)
If Apple supports Vulkan but with worse drivers than Metal, I'm not sure that's any better. OSX has roughly four times the penetration of Linux in the latest Steam hardware and software survey. I'm not sure it's worth it for most developers to deliberately target the second-best choice on OS X in order to pick up the handful of Linux gamers who can't/won't dual-boot Windows.
(EDIT: see below.)
[1] http://store.steampowered.com/hwsurvey [2] https://community.amd.com/community/gaming/blog/2015/05/12/o...
That seems like a far fetched theory to me. AMD and NVIDIA haven't released drivers for XP for ages. Vulkan support in Source Engine is nowhere near done, if it has even started. In the meantime Chinese players will move on...