Question: Sally tries to prevent Jack from taking over Christmas using Fog Juice. How will fog stop him?
Answer: Well, it won't. The fog only covered the ground. Because once Jack flew off in his sleigh, he could see. So, it didn't really stop him. He just had to fly above it, and boom! He could see.
Answer: As seen in the movie, the Fog Juice made it difficult to see anything. Sally had hoped that with no way of seeing where he was going, she would be able to stop him from leaving Halloween Town and save his life.