Hi there. I’ve actually never heard of this experiment before, but I did a quick search on it and it might be that you have to refresh the vinegar every day. So on day one, put the egg in a jar, and fill the jar with vinegar until the egg is covered. Then, leave it for 24 hours, and on the next day, throw away the vinegar and cover it with new vinegar. Then leave the egg and vinegar alone for 7 days.
I think you have to change the vinegar after 1 day is the first patch of vinegar kick starts the chemical process, but it’s lost all of the necessary chemicals after 24 hours, and so you need to refresh it to keep it going.