This is a short video showing the process when troweling the bottom in on a vinyl liner inground pool. You can see how we start with nothing but dirt and gravel, and trowel the base material over that untill the bottom is finished.
We use a vermiculite base rather than a grout base. Vermiculite sets up hard enough to walk on, but stays soft enough to be able to do any repairs needed. We can scrape any trowel marks, or repair any rough patches to get the bottom as close to perfect as possible. With grout (concrete) base you can not do this. Unless if a grinder is used any trowel marks or imperfections are permanent.
Vermiculite also allows water to flow through it back and forth. This allows for it to not crack like a grout mix will.
You can see the string lines we set in the bottom. This lets us trowel the bottom in exact so that our liners fit like they are supposed to. If you do not set string lines you are guessing at the dimensions, and there is a good chance your liner will not fit perfect.