There is little that can be done to repair it permanently. If the vinyl is curling upward, then it is showing its age. Soon after it starts to curl, it is likely to crack also. There is a short-term solution to the problem. You can use an iron on the floor to flatten the curled piece of vinyl, as long as you lay out a towel first. This way the vinyl will absorb the heat and flatten into place, but it will not burn. Once it has been flattened, put some weights or some books over it to ensure that it is flat. While the books are in place, the vinyl can cool at the same time. When it has cooled, put contact cement on both the underside of the vinyl and on the plywood floor. It is important to apply the contact cement on both surfaces, or it will not stick. It is also important not to connect the two surfaces immediately after the contact cement has been applied. Let it dry until it is tacky before pressing the vinyl into place. You are going to have to replace the vinyl within a couple of years, but you will have a flat floor in the meantime.