Lens won't stop up aperture manually?

I have a vivtar series 1 f/2.8 28-90mm lens, nikon f mount. I can get the aperture to close if i push the camera aperture control switch closed, while the manual ring is closed down. It then opens and springs closed like a lens without a manual control ring. But if i open it back up with the manual control, it will not closed again until i push the camera control switch manually. Being as i have a d40, this means i have to dismount the lens every time i want to close down the aperture after opening it.

so, what would cause a lens to open manually, but not close manually, and is there anything i can do to fix it?

Not unless you know what you are doing and have special tools. I would personally take it in to a shop and have them repair it.

That lens comes in either AI or AIs, it won't work with a modern digital camera [like yours].It won't couple to the meter, the camera's pentamirror will not work for focusing manually (because you can't see focus well enough), and apparently yours is defective, or it isn't even an AI. Get a modern lens is how you fix your situation. The cheap amateur nikon DSLR's aren't for people who want to experiment with old lenses. If you insist upon wasting your time with doing it, you need either a Dx or Dxxx series camera.