Vulkan has nothing to do with the Steam Deck being able to run PC games other than being the API that DXVK and VKD3D-proton translate DirectX calls into. Vulkan isn’t the magic bullet, it’s just another API in the stack. The “magic” is the phenomenal amount of effort that has gone into those DirectX translation libraries.
Vulkan makes the work easier, but it is not what makes those games portable.
Yes and no. DXVK didn't happen on top of opengl as the impendence mismatch between OpenGL and DX is too large. Wine had working but slow DX9 to OGL; incomplete, buggy and slow DX11 to OGL; and no hope for DX12 to OGL. DX9, DX11 and DX12 are all much easier (but not easy, mind) to map to Vulkan.
As Steam Deck is running AMD, a conversion layer on top of Mesa Gallium would have been possible, but again DX12 support never materialized.
Vulkan makes the work easier, but it is not what makes those games portable.