It always returns 0.0f for both. I investigated the Engine source, and it looks like somebody forgot to finish writing the function.
void FSteamVRHMD::GetFieldOfView(float& OutHFOVInDegrees, float& OutVFOVInDegrees) const
{
OutHFOVInDegrees = 0.0f;
OutVFOVInDegrees = 0.0f;
}