Here's my 'dumb' question: when the pump and filter are running, is water pulled from the main drain as well as the skimmer generally?

I have never had to drain my pool for any reason, but think I might need to do a partial drain to get my calcium down (it's 450-500). I recently tried draining a little (through a hose connected to the 'faucet' that sticks up from the pump) but I was afraid to go below the level of the skimmer because I didn't want the pump to be sucking in air. Can I safely drain the pool this way, or is there another method I should use?

I have an 18,000 gal inground plaster pool (3 1/2 years old). There are three pipes that lead to the pump - the skimmer, the main drain, and the pool vacuum. There is a Jandy valve for the main drain and I can't tell if it's in off or on position, (it either points up along the pipe as it is now, or turns 45 degrees to the left) but it has not been changed since the pool was built (I am the only one who maintains the pool).

While I'm asking about the drain - if it is running, is there any danger for swimmers in the pool (specifically my 5 and 7 year olds)? How about the vacuum (Hayward Navigator)? I always leave that in while we swim since it's a hassle to take it out, and sometimes it is running.

Thanks for answering all my questions... this is a great place for information!
Nicole