This Original Post is about getting the Vive Tracker to work with OpenXR.
If you are unaware, there are 3 major Input Schema's using SteamVR products.
1. OpenVR
2. OpenVR 2.0 (2018) which adds the SteamVR Input Actions system.
3. OpenXR which is the new standard and uses the Unity Input Actions system.
I am pretty sure that VIU was built using OpenVR 2.0, and maybe has been updated to work with OpenXR in some ways, but most likely doesn't add any Vive Tracker functionality with OpenXR.
If you look at the OP, they share a link to a YouTube video, which currently, is the only way to get the Vive Tracker to work with OpenXR. This is not a HTC Vive implemented way to get Vive Trackers to work with OpenXR, but instead is a custom script written by people on the Unity Forums. So it looks like currently, the Vive Tracker isn't currently officially supported by HTC Vive with OpenXR, only with OpenVR and OpenVR 2.0. It would be great if the Vive Trackers could be officially supported by HTC Vive to use OpenXR, which is the new standard.
I am able to get the Vive Tracker with input working in OpenVR 2.0, but not with OpenXR... so this is what I am trying to get solved here.
I tested what you suggested, installing VIU and running the UGUI scene, but running this makes no sense as it is a scene meant to work with Controllers, not with Trackers. The tracker doesn't show up, but it works with controller, so this doesn't help.