[2021] What is vulcan?

Like DirectX and OpenGL, Vulkan is an API. Vulkan is designed to offer greater performance, so it is generally recommended to use Vulkan when you have performance issues. It usually should improve your FPS.

Edit: If you want to know more, read this article: "Should you play Valheim using Vulkan API?"

Yeah, it totally depends on the system. For some it's good.