Get the smallest vaccum you can find, so you'll have access to all parts of the aquarium. These work by establishing a siphon, so the water is pulled out of the tank and into a bucket or sink that's lower than the level of water in the tank. By running the vaccum through the gravel, loose debris and water is removed from the tank. A line of large clear tubing will work equally as well. You can control the flow rate by closing off a portion of the end outside the aquarium--but it takes practice not to result in a nifty spray.
Snails are not prone to escaping. Apple snails will make forays above the water line, but seldom stay long, and I've never had one leave the tank completely. It is important that they can get to the water surface, since they are air breathers. They have a small tube, called a siphon, that they stick up to get rid of the used air and take in fresh. Mine have the habit of pumping thier shells at the same time--they look rather goofy.