How do you plug a main drain in bottom of vinyl liner pool that is leaking?

If you are talking about,pluging off your bottom drain and just using your skimmer only to run your pool,then you can buy a plug made of rubber or cork to fit inside the hole inside your bottom drain. Just make sure it is in secure. We do this every winter,when we close our pool,because our filter is below our pool. If we don't water will run back in to the pipes and could freeze and bust the underground pipes. We have been doing this since 1977. Good luck.