I think that depends on what executable you are calling. The default shortcut installed with Steam is to some online steam BS. I switched to using a shortcut directly to "bg3_dx11.exe" but I assume you could do the same with "bg3.exe", which I assume is Vulcan. Someone correct me if I'm wrong.
EDIT: Reddit says I'm right:
Location: "By default it's C:/Program files/Steam/steamapps/common/Baldur's Gate 3/bin"
Versions: "Here you should see both bg3.exe (Vulkan) and bg3_dx11.exe"
Here's how to do it directly in Steam, too, if you want: https://eip.gg/bg3/guides/how-to-skip-the-launcher/
EDIT 2: @Hairyblue commented here: "Thankfully, you can use the launch command --skip-launcher to boot straight into the DX11 version of the game, or use --skip-launcher --vulkan to boot straight into Vulkan."