Take down the old vinyl fence. You cannot correct the levelness of the ground without taking the fence down and correcting the problem on the ground level.
Place a construction level used to measure the levelness of interior floors on the ground. A liquid bubble inside the level’s window should move to the center of the window if the ground is level. If it is not, then the bubble will move to the right or left. Your job is to continue using the level until the bubble moves to the center.
Pour potting soil on the unlevel ground and smooth it out with a shovel. Pack the soil down and place the level on the ground. Continue these steps until the level shows the bubble in the middle of the window.
Put the vinyl fence in the ground again.