liner has sunk 4 - 6 inches in deep end corner

We recently installed a 20 x 40 vinyl lined pool with polymer walls and vermiculite bottom. In the 8' deep end of the pool, one of the corners have sunk 4 - 6 inches. :mad:
The pool contractor has said we will need to drain all the water, peel back the corner of the lining, put more vermiculite in and seal everything back up.
Is the new liner going to hold up to all this? Why did this corner sink?
We have had nothing but problems with this pool contractor.
Our concrete guy had to work 4 hours just to straighten the walls because the width in the middle was 3" shorter than the ends which would cause major problems for the underhung pool cover.
We want to hold the money back.....any thoughts on how much it would cost to fix this sunken corner?
We are on week 10 and still not swimming!:mad:

Not to be an alarmist, but that sounds like a MAJOR issue. I would immediately withhold payment.

If the builder dug the hole, it certainly sounds like the soil was not compacted properly resulting in the drop. Whether or not the liner can recover from that much stretch would be debatable in my mind.

More experienced folks than me will perhaps be more optomistic but I'd think about circling the wagons 'til you get a resolution you're satisfied with.