Attach string between two stakes inserted into the ground, one in front of each of the posts that hold a fence panel. Adjust the height of the string to match the height of the bottom edge of the vinyl panel.
Undo the panel screws on the fastening bracket to remove the vinyl fence panel. Typically, the panel screws are on the side protrusions of the fastening bracket attached to each post. Remove the vinyl panel and set aside.
Dig a trench between the posts to the appropriate depth relevant to your slope to house a concrete block retaining wall. Typically, you need a compact 4-inch gravel base plus the height of the concrete blocks and jointing mortar. Ensure that the final height of the wall will be 2 to 3 inches below the string line to provide a uniform gap that most vinyl fence manufacturers recommend you have between the panel base and the ground.
Compact the gravel into the base of the trench with a hand tamp. Shovel 3 to 4 inches of concrete on top of the gravel. Bed each concrete block into the concrete, and use a carpenter's level to ensure the blocks are horizontally and vertically straight.
Mortar each joint between blocks with concrete, using a flat trowel. Strengthen the base of the wall with additional concrete tapered from the side of the blocks down to the gravel. Build the wall in layers to 2 to 3 inches below the string line height.
Allow the concrete to set for a minimum of 24 hours. Shovel gravel into the trench up to the natural ground level, and then shovel soil up to the top of the retaining wall to level the ground. Fasten the vinyl fence panel back onto the brackets attached to each post.