How do I add Vuforia SDK with Unity after the installation of Unity?

Actually if you install unity 2017.2 or newer no need to add vuforia sdk. Just go to file-> build settings-> player settings ->click on vuforia augmented reality

And also in new versions of Unity (e. g. 2019.2.10f1) it is File > Build Settings > Player Settings > XR Settings > Vuforia Augmented Reality Supported checkmark

This will automatically import vuforia related assets into unity.

Install the latest version of Unity, and in the Unity component selection section of the installer, select Vuforia Augmented Reality Support, along with either the iOS Build Support or Android Build Support packages.

Create a Vuforia developer account from the Vuforia registration page. This account gives you access to the tools you need to make AR and MR applications with Vuforia in Unity.

If you have not already created a Unity ID, then do so from the Unity registration page. You need a Unity ID to download any packages from the Unity Asset Store.

Open Unity and create a new 3D Project (make sure the 3D option is selected next to the Add Asset Package button). Name your Project, then click the Create project button.

To activate Vuforia in your Unity project, access the Player settings from Edit > Project Settings, then select the Player category, and select the tab for the mobile device you are building to. Under the XR Settings panel, enable the Vuforia Augmented Reality Support property.

From Unity Docs

Since 2019.3 Vuforia is not native in Unity 3D anymore:

On Friday January 24th Unity posted an “XR Platform Updates” article. Unfortunately, the original version of this article included unclear information about Vuforia Engine. Vuforia Engine is still supported, and will continue to be supported, in Unity. What has changed is that the Vuforia Engine package will no longer be natively distributed by Unity but can be easily obtained from the Unity Asset Store via our Core Samples or our hosted NPM registry. You can learn more about how to download the Vuforia Engine package here.

We apologize for any confusion. If you have any questions, please feel free to post them on our community forums.

Thank you,

The Vuforia Team

We have to download an external SDK again: https://library.vuforia.com/content/vuforia-library/en/articles/Solution/vuforia-engine-package-hosting-for-unity.html