As you may know, Vulkan is the low-overhead, cross-platform successor to OpenGL, a dated API created by Silicon Graphics way back in 1992. The Khronos Group obtained the standard in 2006 and has been working on its successor ever since, unveiling its new API at GDC 2015.
Vulkan gives developers direct access to the GPU and offers more efficient multi-threading capabilities. For the end-user, this translates to better overall performance and in the case of Doom, it will allow those with older video cards to run the game at acceptable framerates.
The Talos Principle was the first game to support Vulkan followed by Dota 2 in May. That said, id Software considers Doom to be the first AAA title to use Vulkan with the developer saying it has worked with both AMD and Nvidia on a daily basis since late March to bring full Doom and Vulkan driver support to the community.
To enable Vulkan in Doom, navigate to the advanced settings menu and switch the Graphics API from OpenGL to Vulkan. You’ll be kicked back to the desktop briefly as the changes are being made. id Software CTO Robert Duffy does warn, however, that there may be a few bumps but added that their testing is showing really great performance and stability.