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?"

It is also less stable than Dx though, I had 3 crashes playing Valheim in my 280 odd hours, and all have been with Vulkan.

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

Originally posted by Flare |UKCS|:

It is also less stable than Dx though, I had 3 crashes playing Valheim in my 280 odd hours, and all have been with Vulkan.

I can say quite the opposite. On my potato PC with glitchy old 2Gb RX460 Vulkan give me pretty stable game without crashes and incredibly consistent FPS. Unlike DirectX, where FPS can jump every second, Vulkan is holding the mark.

My rig is 5 years old, a i74970k and a gtx1070, while Vulkan did seem to perform slightly better... all my crashes was with Vulkan.
I did note though oddly, the Vulkan API was stable using the HD Overhaul mod... and the 3rd crash occurred when I reverted to the default graphics as I prefer them.

Vulkan hasn't caused issues for me and it doubles my FPS so I always pick that launch option.