Hello,
Unfortunately I do not own or have tested vuforia, but by taking a look at one of their tutorials, it seems that they suggest to use an existing ARCamera prefab. Since though Slate is rendering cutscenes from within a dedicated “Render Camera” (found under the “Director Camera Root” gameobject, using this ARCamera prefab will not work out of the box.
What you can try to do, is to add this ARCamera prefab under the “Director Camera Root” gameobject and delete the existing “Render Camera” object (thus basically replacing “Render Camera” with “ARCamera”).
This is not tested, but it should work considering I understand how vuforia works correctly.