A 15lb bag should be plenty of sand. I'd highly recommend washing the sand before you put it into the tank, onyx sand has a lot of black powdery particles in it that will really cloud your tank up. An easy way I found to clean the sand was to get a large bowl, put some sand into it (half full or so) then fill the bowl with water. Now swish the sand around with your hand and just when it settles, pour the water off the top of it. After a few times of this, the water will remain clear and you can dump the sand into the tank. It's a bit of work, but well worth the effort ... otherwise your water will be black cloudy. Doing this will also keep the sand from clouding up your tank each time it's disturbed once you get it all setup as well.
When I vacuum, I just barely glide the vacuum over the sand. Nearly all of your gunk at the bottom will be in the very top layer of the sand, so there is really no need to dig down into it for cleaning.