Adding VIVE Registry to Your Unity Project
Unity transitioned to
package manager
for better modularization and package management support. The package manager gets packages from registries. By default, Unity package manager connects to Unity official registry. Meanwhile, Unity provides
scopedRegistry
support for 3rd-parties to set up their own registries.
Note: the most recent Unity LTS (Long Term Support) is recommended (2020.3.x LTS)
To let your project access packages from VIVE, you can add VIVE registry by either of the following methods:
Adding VIVE Scoped Registry from Edit->Project Settings->Package Manager and add following values:

Name: VIVE
URL: https://npm-registry.vive.com
Scope(s): com.htc.upm
Then, open your Unity package manager window, you can browse packages from VIVE in it.
Adding via VIU (Vive Input Utility)
If using the VIU cross platform utility selecting Wave as the target platform will automatically add the VIVE scoped registry.
https://assetstore.unity.com/packages/tools/integration/vive-input-utility-64219
https://github.com/ViveSoftware/ViveInputUtility-Unity/releases
Showing Preview Packages
If you want to see packages in preview, remember to check
Show preview packages
for the package manager settings window as shown above (same with showing dependencies).
Switching to My Registries
Only in Unity 2020.1 and newer, packages from scoped registries will be listed in another place called
My Registries
.