I have a vintage clip earring that has lost the tension to stay on the ear -How can I repair it?

Another possibility is to add tiny magnets.
You can use a clip earring tension adjuster called Clip-Kease. Looks like a key and it will loosen or tighten clips for earrings.
Clip-Kease and other earring problem solving products can be found at http://www.earringdoctor.com/clipkease.html