I am not a liner expert, nor I have replaced a liner before, but I myself and my inlaws both have vinyl liner pools. While our liner is still secured in the track all the way around the pool, theirs has slipped out in one section about 4"-6" long and I can tell you the liner has sagged down in a semicircle in that section. While it does not look good, it has not seemed to have gotten worse and does not appear to affect the operation of the pool it is something I would avoid it possible. Since it appears your pool is empty and you are installing a new liner I think it would be prudent to try and fix this problem now. If it is not taken care of now just think of how much work it will be to fix it once the pool is full. It seems like this would be a semi-common repair and someone should be able to give you some advice on fixing it.