I decided to tackle your problem in Construct 2 and try what seemed like the simplest solution to your problem and set up multiple backgrounds with the bullet behavior and the wrap behavior. The idea is that only one would switch to the other side of the layout, you would see the other one moving up and give you the illusion that the entire level is moving vertically because the backgrounds are infinitely looping.
As you can see, I tried to get the to loop, but even though both scrolling backgrounds were right next to each other, there was a gap between them.
Link to the demo on Google Drive here: drive.google.com/file/d/0B1VrguiWkW
C6cDU1ejBtcE4yS0k/view?usp=sharing
In other words, we need to find the solution to this problem if we want to make a working veritcally scrolling background.