So here what I've been thinking, at the end of the movie, Ralph said that when the nicelander lift him at the top of the building he can see Vanellope in the Sugar Rush acarde.
So there might be a chance for getting the code to unlock Vanellope in the game if we give Ralph an opportunity to see Vanellope.
But still, that's only my guess. I made it to level 34 and still no sign of stopping. I don't know if the web version is endless or not. If not, there is a pretty big chance that the reward is the code for Vanellope. Please comment your opinion.