How do you fix a sunken in areas in the bottom of a vinyl liner?

I have only done this one time. I branch fell from a tree in a wind storm from about 30 feet up. Went through the winter cover and through the liner in the deep end of the pools. It was stuck there when we opened the pool in the spring. The branch was about the size of a Baseball bat. I had prepared a zip lock back with play sand and also had a patch already cut. When ready, I removed the branch, placed the ziplock bag under the liner in the hole and then proceeded to patch the hole with a vinyl patch. If this is in an above ground pool, I wouldn't mess with it unless you think it is going to burst. Once you cut the liner more water is going to push out and make the sunken area even bigger. For above ground pools theres not a whole lot you can do. Maybe get rid of the moles....