Surface prep for vinyl tile

One option is to use a straight edge and cut back to a sloid point of flooring. There is a product on the market (i can't remember the name) which is a self leveling liquid. You can then pour the liquid in the void area and it will harden for you. The product is specifically designed for applications such as this. Just be sure to take the time and level any other areas that might have deteriorated over time, and inspect the floor for any other areas that might need to be cut away. I will be heading by the home depot later. If I see the product name, I'll be sure to post it. I have used it a couple times and never had a problem.