Vuforia has a memory leak on iOS when the setting for the Target Resolution is not set to native.
For a while there has been an issue with the screen not filling properly under these circumstances, which could be fixed by editing the XCode project - however the memory leak is a complete killer.
You can verify this by using the FrameMarker sample and setting the Target Resolution to auto, best performance. Running with the Memory Monitor instrument will reveal a constant and fast increase in the Real Memory used by the application. Returning the setting to Native will remove this leak.
Thanks for the update Stefann - glad it's sorted for you.
It sounds like a similar problem to the Image Effects issue posted elsewhere- when a developer tried this it was leaking memory, but the 4.2 update fixed it :)
N