I would not completely drain- 2/3 is max I would do (I would probably only do 1/4 at a time, multiple times- but my well is good, so I got plenty of water with no metals/etc.)
As far as sump pump- make sure it has an adapter on it so it doesn't suck up the vinyl, and it should be fine on the pool floor.
What happened to the warning about another post? my post went straight thru, didn't know Ben had answered already; didn't mean to add to the real expert!
but like Ben said, don't let the suction sit directly on the floor!