Catalina, love your name :]
My method is very barbaric. Every week I place a square or pantyhose over the mouth of my snail vase and slowly tilt it to release all of the water. The snails that are not left clinging to the glass are washed into the net (pantyhose). I pluck them out of the gunk that washed out and place them back in the vase. You don't have to worry about snails out of water like you do fish. They're fine for a few minutes and will not suffer w/o water. Then I add a bit of water conditioner to the water I fill the vase back up with. I used to use tank water to fill the vase but the snails are such dirty buggers that I use fresh conditioned tap water now. They're quite hardy and haven't suffered. This surely isn't the best method, but it's what I've come up with and is sufficient.
As for where the snails lay eggs, mine have all laid them right near or in between the water level. As in half the little gel sack is below water and half above. If the water evaporates and the sack is completely above water then it will dehydrate and won't spawn, so keep the water level up.