How does setting -vulkan in launch options for games that support it work?

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Is it forcing the games to use Vulkan API or simply the DXVK layer?

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Steam Client just passes command line parameters to the game program. It's up to the program to interpret the options.

If a game supports -vulkan then it means it accepts that parameter and enables its own Vulkan renderer.

DXVK is a series of DLLs that replace the functionality of DirectX and are typically placed in a game folder or bundled with the Proton runtime on Linux to convert Direct3D API to Vulkan.

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So if I run Left 4 Dead 2 with -vulkan in the launch options, I should get better or worse performance in Windows?

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Originally posted by NothingIsHalal:

So if I run Left 4 Dead 2 with -vulkan in the launch options, I should get better or worse performance in Windows?

If L4D2 supports that option it just means switching the graphics API. Whether it is faster or slower depends on hardware and drivers.

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I have Windows 11, i5 9300H, GTX 1050, UHD 630, 3GB GDDR5, 16GB DDR4, 500GB SSD, DirectX12, Vulkan V1.3, FHD LED IPS. I know that Vulkan can run games better than older DirectX, but not sure if Nvidia can render Vulkan better than AMD. I do know Left 4 Dead 2 runs better on Nvidia because it has some licensed code from them lol. And also I get better performance over someone with similar setup to mine albeit with AMD hardware (Windows 10, R7 3750H, RX 560X, RX Vega 8, 4GB GDDR5). But that comparison is running the game via DirectX9C; maybe with Vulkan, the AMD equivalent to my PC would run it better.

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Ok I just tried using -vulkan in Left 4 Dead 2 a few days ago, and it didn't work as I expected. Fullscreen mode doesn't work at all as it can't be selected; doing so just glitches the main menu and when I force quit and restart, I get d3d9 error and game closes. So I'll have to use DirectX9C or not run the game in fullscreen if using -vulkan launch option. I later removed that and decided to run the game in DirectX9C for now.

I also checked and Left 4 Dead 2 uses an outdated version of DXVK; I have the latest version of DXVK Async (variant that compiles shaders asynchronously for better performance) I downloaded from GitHub. But I'm not quite sure if replacing core game files in my context would make me get VAC banned. I have asked Steam Support a few minutes ago, but I obviously gotta wait maybe a few days to get a response.