Ok, so the pump hoses run downwards towards the pool? You filled the dirt around the pool and made it underground sort of? You need to maybe fill the pipes slowly to get all of the air to come out of the high side of the lines before hooking the pump up to them. When you vacuum you may be pulling the air into the pump and losing prime, and since air compresses differently than water, it will do that hissing thing.
I have installed alot of watermain in my time, and it is basically the same theory. Good luck.