If you have a mysterious stain on your vinyl floor and it has gotten larger over the past several years, you may have a classic case of “bottom-up” staining—the stain is penetrating the vinyl from underneath. It may be the adhesive that was used to adhere the vinyl to the concrete, or it could be the result of moisture rising up through the concrete.
If the stain is yellow, it's probably an adhesive stain. If it were black, gray, blue or pink, it would indicate mold growth. Then you'd have to solve a moisture problem before laying your new floor.
You have two options. Either rip up the flooring and scrape off the adhesive, or install a new floor (vinyl, laminate, carpet, wood or floating floor) right over the top of the vinyl, provided the old vinyl is well adhered. If it's coming loose, tear it out and start over. Otherwise, the second option is quicker and easier. Besides, the old vinyl will make a great underlayment. The stain won't come up through the old vinyl and discolor the new floor.
Bottom-up staining has become less common over the last 10 years because adhesive formulas have improved.