Lenovo Z70-80 laptop, Ubuntu Gnome 16.10, Nvidia GeForce 840M GPU, PRIME set to use NVIDIA.
Upgraded driver from 375.39 to 378.13 using https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa
All Vulkan applications (my own, Vulkan SDK samples, etc.) stopped working. Tracked it down to vkGetPhysicalDeviceSurfaceFormatsKHR. It returns VK_ERROR_INITIALIZATION_FAILED, which, according to the spec, shouldn’t even be among the possible return values: https://www.khronos.org/registry/vulkan/specs/1.0-wsi_extensions/html/vkspec.html#vkGetPhysicalDeviceSurfaceFormatsKHR
All functions called before this seem to return VK_SUCCESS.
Downgraded back to 375.39 and things work once again.