but that is age, leaching of stuff from the sub flooring, and just lack of periodic cleaning the rest was able to have.
IF, the good vinyl tile is still around, then go and re tile that one area, you will still have a slight discoloration from new to old.
My best suggestion would be to completely re tile the kitchen area. I would imagine that the floor would be in good enough condition to go right over this. Stagger the tile laying so that the tile are off set from the base.