O,
Well, where do you plan to connect the vacuum?
It is impossible to connect it to the main drain, and that only leaves the skimmer. If you shut off the skimmer, you will have no suction going to the vacuum.
Leave the skimmer and main drain wide open. Add your vac hose to the skimmer and start the pump. Slowly.. close the main drain valve a little at a time. Stop when the pump starts to struggle because it is not getting enough water and then open the main drain back up just enough to keep the pump running well.
If you find that you can close the main drain and the pump is still running well, that is great!! Just leave it closed.
Thanks,
Jim R.