Sub Floor re-fixing
however you mention the floor is not level and that would be a problem.so the question is, what is the cause of the floor not being level?will pounding the nails down bring the floor to level? if so no problem. however if the underlayment is the problem you will need to pull it up.then if the plywood is level ,lay on that.....good luck
Most vinyl layers that I know use an air stapler and staple down underlayment and they use a lot of staples. You can rent one if you don't have one or buy one. They come in handy for a lot of other things than just underlayment ( almost wrote "nuderlayment"). You just have to try a few to get the air pressure right so you are setting the staples but not so that you are blowing clear through it.