Fog in the lense of
The fog is most likely lens fungus and is very difficult to remove, so is also expensive to remove. It's not suited to a DIY repair without special tools and equipment. Spare parts for your lens are practically unobtainable, and camera and lens repairers are much rarer these days.
With all due respect, your lens is nearly worthless even if in perfect condition, so your lens is totally beyond economic repair.
You have the following options (in no particular order):-
1. Discard your lens and replace it. You can find replacements very cheaply online, although you may need to be a little flexible as to what's acceptable. Tamron Adaptall-2 lenses can be very good and you can switch the lens mounting to fit your Minolta MD. Lesser-regarded brands such as Sunagor were also in reality very good lenses and their 80-250mm has a very similar effective focal length to your Vivitar. Hoya were also a less-regarded brand despite actually making many of the lenses for "better" brands (including Vivitar, from time to time). You can also find replacement lenses for free either by asking around friends and relatives or by looking/asking on FreeCycle and Gumtree, although it is harder to find them for Minolta than say, Canon, Nikon or Pentax. On the flip side I have had many complete and perfectly functional 35mm SLR outfits from FreeCycle, so if the lens comes with a body attached who's complaining?
2. If the fogging does not affect your images then live with it. if it does reduce contrast, then consider scanning your negatives/transparencies and using free software to repair the images.
3. Leave your lens out in daylight with the lens cap removed. Some of the common types of lens fungus are destroyed by UV light. It won't physically remove the fungus, nor will it repair any damage to the optical elements inside your lens, but it can recover an otherwise unusable lens. If the fogging inside is due to moisture instead, then the fogging may dissipate if the lens is left in a warm and sunny location which is well ventilated. Marks may remain, but they'll rarely affect image quality.