A question for experienced players: what is this, how does it work, and should I trust it? Dx9 > Vulkan

I don't know what that file is, but it's most likely DXVK.

You can download the DLL from their official GitHub page (https://github.com/doitsujin/dxvk/releases)

I installed it when I needed to enable effects in Reshade for Heroes, which DirectX 9 didn't support.

Otherwise, without Reshade, there was no difference from changing the renderer...