Doesn't work with Vulkan

Vulkan apps always run on my Nvidia GPU. See this shell session:

$ switcherooctl list
Device: 0
  Name:        Intel® UHD Graphics
  Default:     yes
  Environment: DRI_PRIME=pci-0000_00_02_0

Device: 1
  Name:        NVIDIA Corporation GP108BM [GeForce MX250]
  Default:     no
  Environment: __GLX_VENDOR_LIBRARY_NAME=nvidia __NV_PRIME_RENDER_OFFLOAD=1

$ switcherooctl launch --gpu 0 vkcube
Selected GPU 0: NVIDIA GeForce MX250, type: 2

$ switcherooctl launch --gpu 1 vkcube
Selected GPU 0: NVIDIA GeForce MX250, type: 2