It appears updating the Nvidia 410 or 415 drivers to the latest version from the Proprietary GPU Drivers PPA broke Vulkan for some users.
After updating either to Nvidia graphics drivers version 410.78 or 415.18, some Ubuntu (but Linux Mint and other Ubuntu-based Linux distributions users are probably affected as well) users are reporting that Vulkan no longer works, breaking any game (for example any DXVK / Proton game) or application that was using it.