Just look at the Steam interface for OS X. They haven't even updated to the new design that Yosemite brought. Then you google Valve actually 'trying' to improve Mac gaming experience and you get this...
https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=valve not optimized for mac
Valve's income and user base has never been because of Mac gaming so in the end it kinda makes sense that they haven't done anything to improve the experience.
Also your post made me laugh a bit. Just because Vulcan might be 'similar' does not mean anything. That's like saying because Swift 2 went open source...its now like this other language. I also wouldn't call Valve and Apple being in a relationship, when was the last time they worked on something together? Steam for Mac? Did Apple help Valve with the Steam Machine? In reality, Valve cares about PC gaming and Apple cares about making hardware and software for people to use. Mac gaming is not something they really work together on so in the end, Apple has no leverage on Valve. In fact, Valve probably has the leverage on Apple if Apple ever wanted to get serious about gaming.
Also realize...they are two competing APIs. Why the hell would Valve bow down and kiss Apple and just throw away their own?
Sums it up perfectly.