The Vulkan performance on Dota 2 is lower than with OpenGL, as many have already found out. There's also some other odd NVIDIA driver happenings as shown from the comparison results on the previous page as well as the Vulkan renderer hitting problems with the GTX 950/960. Though in due time once Valve continues optimizing their Vulkan Source 2 support and NVIDIA/AMD continue with their Vulkan driver optimizations, the situation should be much more interesting. It's great to also mention that the CPU usage is noticeably lower with the Vulkan renderer already compared to OpenGL. [With Dota 2 on OpenGL, the CPU usage was around 28~35% while switching over to the Vulkan renderer put the CPU usage at 14~20%]
Now before you say "well so what?", this is the same kind of debate which is going on right now with Doom 2016 about its Vulkan performance, as well as other games which use Vulkan instead of OpenGL. And, as with OpenGL and DirectX, it's not some miracle product, it requires developers to work hard to fine tune it or performance is going to be in the toilet.