Not sure if this is right but from what it looks like (with whatever video editing software they used) is that they made the size of the whole video lets say 1366×768px with a background set as green, and the size of the vine 1000x768 (or whatever) and place it in the middle and wrote text on the sides. So technically made the vine smaller than the actual size of the video, put a green background, and placed it in the middle. I can try and make a little tutorial if you'd like?