InputManager isn't well suited for this task, I strongly recommend interfacing directly with the APIs that
SteamVR provides for Unity games:
SteamVR_Controller.Device.GetAxis is most likely going to be the easiest way to get the behavior you want:
public Vector2 GetAxis(EVRButtonId buttonId = EVRButtonId.k_EButton_SteamVR_Touchpad)
So to treat it like a joystick (for the most part):
// I assume you have a SteamVR_Controller.Device named `device` available:
// Is the touchpad being touched?
// This will fetch the Axis of the Touchpad by default.
Vector2 touchpadPosition = device.GetAxis();
// this is nearly identical to the behavior of Gamepads
// treat touchpadPosition like the vector form of:
// float x = Input.GetAxisRaw("Horizontal");
// float y = Input.GetAxisRaw("Vertical");
GetAxisRaw instead of
GetAxis as an example because SteamVR will handle smoothing and ranges slightly differently than the Unity default inputs, but you should be able to work around it and decide how you want to handle smoothing and dead-zones yourself.