Seems like not even a simpliest application is working with Vuforia 4.2.3 and Unity 5.0.1p1. Object tracker + AR camera + cube = UI as in 2nd image.
---
Some more testing showed that with Image tracking the same issue appears.
---
Using Screen Space - Camera instead of Overlay seem to solve the problem
Can not recreate the issue of font going square. Anyone from Vuforia support aware of these issues?

See this post:

https://developer.vuforia.com/forum/unity-3-extension-technical-discussion/vuforia-42-unity-501-ui-issue#comment-2050105

Is this the solution?

"Using Unity 5.0.1p3 and Vuforia ImageTargets 4.2.3 sample, I created a Unity Canvas element set to Screen Space - Overlay along with a Text element and it displayed fine on an iPhone 6 (8.3) device."

I've been able to get all 3 Unity Canvas modes working with Unity 5.0.1p3 and ImageTargets 4.2.3. Are you using Unity's UI or a 3rd party plug-in such as NGUI? If you are using Unity's UI, are you able to provide steps to reproduce the issue using the ImageTargets 4.2.3 sample which we can try?

We are using Unity 5.0.1p4 now and steps are:

1. Import Vuforia package
2. Drag and drop AR Camera and Object target. (set up license, import tracker data etc.)
3. Put a cube on object target
4. Create Unity UI with vertical group containing 3 buttons having layout element component on them.
 

That's it.

Hi,

I just tried the same test, but it seems to work fine (I can see the 3 buttons in my vertical layout panel), both in Camera and in Overlay modes;

using Unity 5.0.2 p1 (tested on Android and iOS 8.3 devices)

See attached screenshot taken from iPhone 5S running iOS 8.3

See similar issue (likely same issue) here:

https://developer.vuforia.com/forum/unity-3-extension-technical-discussion/vuforia-42-unity-501-ui-issue