We built vinyl pools for many years. The steel panels all join at a 90 degree angle in the corners. For a common 6" radius corner, a filler piece is added in each corner to create that radius. The liner track that fastens to the panel top can be purchased pre-curved for that radius or a notched track can be used.

We have built some with a 2' radius corner which is a separate corner piece all together. The large radius makes for an easier liner install.

90 degree corners make it difficult to keep the liner in place-lots of tension.

The coping stone always follows the contour of the panel. Your contractor used a 6" radius corner and had the liner made for that, but the coping was set at a 90.

I would not be concerned about water getting in as it will get in around the remaining perimeter of the pool. It just looks crummy with that gap.

You could fill with a colored silicone but it may make it stand out even more.

Bottom line is the contractor should have cut the coping to the follow the radius.

On a small radius like this, the cuts to make the corner would be individual small pieces.

Having 4 custom stones fabricated in 1 piece would look nicer. This would require removing the 2 stones that make up the present corners and fitting in new pieces.