I've run into a strange issue that seems to happen only with Vuforia 2.5. Here is a link to a zip of a simple project demonstrating the bug (too large to attach), and I will attempt to explain it in some detail:
In my scene, I have three cameras, an ARCamera and two regular camera. The first regular camera, let's call RenderCamera, is rendering to a RenderTexture, which is on a plane visible only by the other regular camera, let's call UICamera.
The scene plays as expected in the editor using the webcam - I see the video feed with the camera plane visible. However, when it is built to an iOS device, something bizarre occurs. The portion of the screen that should be the video feed goes black, and the RenderTexture plane is filled with the video feed, almost as though it is working as a mask. The cube in my scene shows up in the expected position over the black ARCamera portion.
The same project using Vuforia 2.0 appears to work as expected.
This bug also occurs if I delete the UICamera and allow the camera plane to be visible to the ARCamera.
This project was created in Unity 4.1.4.
Thanks in advance - let me know if you have any thoughts.