cracks in concrete

[Jeffski]

It's a totally different product for a liner pool.

[KirstenHW]

Our vinyl pool has a vermiculite bottom - more forgiving (and gentler on the liner) than straight concrete, way sturdier than sand. Installed in 1982, liner was replaced in 2004 - and there were no cracks in the vermiculite to be repaired after all that time. We had 2 divots in the shallow end they patched up - one caused by the ground settling, the other from a kid jumping into the shallow end (another good reason to only jump into the deep end).

You need to have the contractor fix that immediately - it will definitely show under the liner, probably will damage it in time and if you are getting a light, it will stick out like a sore thumb when lit up.