Is it the original material on the seat?
In a situation like this, you would remove the seat cover, split apart the area that needs to be replaced, sew in a new piece of cloth, sew the whole thing back up and replace it on the seat.
The reason i asked if it was original (34 years old), is that sometimes vinyl may look OK, but when you start to mess with it, it will crack or split apart. Vinyl can get sort of brittle with age. If it is the original material, I would plan on getting the whole seat re-upholstered. Chances are the vinyl is pretty fatigued, are should be replaced also. Put it this way, if a seat such as this came into my shop, we would only recover the entire seat. You would be looking at about $125-$150 to patch the seat, and about $300-$350 to recover the whole seat, plus materials. The way I see it, the extra couple hundred bucks is worth getting a brand new seat cover.