OpenVR only works with DirectX 11?

Whenever I disable "auto graphics API" for Windows, and try to put either Vulkan or DirectX 12 at the top, I can't get my game to play in SteamVR. If I set "auto graphics API" on, my Vive works fine with the game, but only in DirectX 11. I'm testing outside of the editor with proper built EXEs. Windows 10 x64. NVIDIA 1070, March 20th drivers.

Unity v5.6.0f2

I see there is a line item for initial support for Vulkan OpenVR, and I thought DirectX 12 was already implemented. Should I be able to get OpenVR/SteamVR to work in built Unity 5.6.0f2 projects with something other than DirectX 11? If so, what am I doing wrong?

OpenVR is at the top, and Virtual Reality Supported is checked. SteamVR is running in Steam.