How can I install a fence on top of this wall?
A somewhat destructive and costly alternative:

Use concrete anchors to fasten 4x4s to the lower portion of the wall.  Cut the 4x4s so they are the same height as the wall to the right in the pictures.  Then you can lay a trellis, slats, or whatever across the 4x4s using screws, bolts, etc.  This will likely ruin your lovely greenery and cost more than your c-clip idea, but this might be sturdy enough.

Can't tell from the pics, but I'm assuming the ground that meets the fence is composed of those concrete blocks?  If there is dirt right up against the fence, drive the 4x4s in the ground for more support.

Alternatively, hammer off the sloped top of the wall so it is flat, and fix cinder blocks as high as you like.  This might actually preserve your vines.