Can you use cement to repair in ground pools that have a vinyl liner?

You absolutely can install concrete under a vinyl pool liner...however, if you have ground water it would not be a good idea due to hydrostatic pressure which can cause the concrete to heave and lift. Additionally, concrete is inherently rough, where other produces such as vermiculite (poolkrete) or a sand/portland cement mixture will serve as a more even surface. In my opinion, I would use poolkrete as it provides the ability for ground water to travel through it as well as providing a smooth surface and insulating properties. Good Luck!