https://np.reddit.com/r/DotA2/comments/4jzsir/dota_2_update_main_client_may_18_2016/
For those of you who don't know what Vulkan is: it's sort of similar to DirectX 12 with the main emphasis lying on drastically reduced CPU overhead and more direct coding to the GPU for dat increased performance. One advantage Vulkan already has is that it supports way more platforms than DX12 with Windows 7 and up and Linux (sadly no OS X yet) whereas DX12 only supports Windows 10. Valve is very active in Vulkan's development, and they've said Source 2 would get Vulkan support as soon as Vulkan's out. Now that Vulkan's released Source 2 gets the API option probably some time next week, assuming Valve time does not apply here.
From Fletcher @ Valve
We're going to release vulkan support soon, probably sometime next week. That DLC checkbox is an error and shouldn't be visible yet. Sorry for the confusion.
For those of you who don't know what Vulkan is: it's sort of similar to DirectX 12 with the main emphasis lying on drastically reduced CPU overhead and more direct coding to the GPU for dat increased performance. One advantage Vulkan already has is that it supports way more platforms than DX12 with Windows 7 and up and Linux (sadly no OS X yet) whereas DX12 only supports Windows 10. Valve is very active in Vulkan's development, and they've said Source 2 would get Vulkan support as soon as Vulkan's out. Now that Vulkan's released Source 2 gets the API option probably some time next week, assuming Valve time does not apply here.