Send the lens in for a service, if it's still serviceable - I don't know how old it is. The chances are there's no way to loosen or lubricate the aperture blades without dismantling the lens. This is a rather daunting task, and when attempted by someone who thinks they're a bit handy with a screwdriver it generally ends in abject failure.
Maybe somebody will come along and suggest a reputable company that can service Vivitar lenses.