This could be a couple things 1) the subfloor is delaminating, there isn't a real easy cure for this. 2) There is an air bubble trapped between the subfloor and the vinyl. You can start by poking a small hole in the corner of a square in the floor, and letting the air out. Heat the spot with a hair dryer and put some weight on it. Get a syringe and inject glue in the bubble.