Since there are no leaks in your equipment, it can only be one of two things, either someone is stealing your water while you sleep, or your liner has a leak
No, you do NOT need to drain the pool EVER! All vinyl pool repair kits come with an underwater glue, there are many glues you can use, the ones supplied aren't necessarily the best, but will do the trick. Just follow the instructions, and buy the best kit you can get your hands on.