I'd like to weigh in, if I may?
Seeing as the lens is a Vivitar T-mount lens, the Rebel T3i has no idea what lens is connected to it (or that a lens is even connected). Even if you were to use an EF-adapter, my guess is that there are no electronics in this lens to communicate with the camera. In most cases, the camera will read "F00", like Wadizzle said above. That means either no lens is attached, or the camera doesn't recognize the lens.
We obviously haven't tested these lenses, so we can't guarantee anything, but we've found that they work best in fully manual mode, so you can tell the camera what you're doing. Remember, the camera doesn't even know a lens is attached when you're looking through the viewfinder.
On the flip side, when you use Live View, the camera's electronics doesn't know about the lens, however, it can tell optically that *something* is there. That means that it will expose the shot to the best of its ability.
That's why the "automatic" settings will be all over the map, and remember, we guarantee nothing when you're using a non-Canon, non-EF/EF-S lens.
I hope that helps!
