Hoping someone here could give me some insight on what makes the Canon one work almost 3x the price of the 3rd party Vello one? Are there compatibility problems?
Well, if there are any problems or it damages your camera, Canon will probably backup the warranties and fix the camera. If you get the Vello they'll charge you for sure. Also with Canon you can be pretty darn certain about the quality and materials used and compatability.
Not saying you should 100% get the Canon grip, but with the Vello, as good as it is, there's less certainty than with the Canon. Plus, is the Vello as well weather sealed and forms a good seal with the body as the Canon one? I dunno, haven't tried it or read any of the reviews. Might easily be just as good, but then again might not be.
If you're planning on just using it for the dual batteries and to add weight/heft and the portrait mode controls and won't be using it any crazy situations, might as well get the Vello since it seems well rated. If there's some question that you'll use it in some inclement weather, I'd go for the Canon.