IG Vinyl Substrate Repair

This is our third liner replacement in 21 years of operating this pool. I have noticed that the bottom of the hopper and the shallow end has sunken a few inches over the years. Water remains in the shallow end and the main drain is higher than the lowest part of the pool. I would like to raise the levels of these areas before the liner is installed. The measurements for the liner were taken at areas where the level appears to be correct. On a previous liner replacement, tree roots were removed and the substrate was repaired. I found out that pool-krete was the typical finish for lined pools and used that product for the repair. Now I am thinking that the original substrate was a sand/portland mix. Is it okay to go over a sand/portland mix with a vermiculite/portland mix? Also, if the area that is being brought back to level is greater than 2", can sand or a different type of material that cost less be used? Knowing that ground water is the reason for the depression of these areas, I have installed a dry well to help alleviate the situation. Any other suggestions or tips would be helpful. Thanks!