Portal 2 to date has supported OpenGL on Linux while Direct3D 9 has been available on Windows for this Source Engine game. Now though Valve is introducing Vulkan support not by moving to a newer version of the Source Engine but in integrating DXVK natively to allow optionally translating the Direct3D 9 path atop Vulkan.
With the "-vulkan" command line switch, this Vulkan-based renderer can now be enabled as an alternative to OpenGL. I'll be running some benchmarks on it shortly.
Thursday's Portal 2 update also makes the game HiDPI aware, smarter default video settings, various rendering optimizations and other improvements, and a whole lot of bug fixing.
More details on this hefty Portal 2 update via the Steam Store .