So I ran into a major issue with Vuforia 5.0.10. In Unity 5.3, UI masks do not work with Vuforia 5.0.10. UI masks worked fine with Unity 5.2 & Vuforia 5.0.6, but something must have changed with Vuforia's ARCamera in the latest release (5.0.10).
Just to make sure the problem wasn't related to anything else, I created a brand new empty Unity project, imported only Vuforia, and created a UI image & UI mask in the scene. Then I dropped in the ARCamera prefab and ran the scene. The UI mask & image worked correctly in the scene view, but in the game view, the UI mask does not mask (hide) the UI image like it is supposed to do. If I turn off the ARCamera, the UI mask works correctly. I have uploaded the test project in case the Vuforia devs want to try it for themselves: https://www.dropbox.com/s/eac6d7vspcphwns/Unity5-3-1_Vuforia5-0-10_maskingTest.zip?dl=0. Or you can build your own new project using the steps mentioned above.
Any thoughts as to how this can be fixed?