The Vivitar35ES is identical to the german Revue400SE. Some of these suffer from gummed aperture blades. Remove the front lens cell and drip some drops of lighter fluid on the aperture blades.
If there is no change the problem is deeper in the mechanism. I once found a severely misbent aperture spring inside the lens barrel, I have no idea how it became misbent but noone had worked on that camera before.
