Shut the pump off. Aim the returns up so the flow won't stir things up down deep. Set the vacuum in the pool. Go clean the bathrooms or something to get ready for the party and let the stuff settle as long as possible. Then start the pump and connect the vacuum and see if you can't sneak up on the DE drifts.
Also consider what I do when I vacuum. Move the valves so you empty the spa into the pool. Then when you vacuum, return the water to the spa. I can usually get the whole thing vacuumed before any water comes over the spillway. There will be no annoying currents to push stuff around, just what the vacuum itself creates.
Whatever doesn't come out is unsightly but not especially harmful. Only the dry DE powder is an inhalation hazard. There shouldn't be enough stuck on people to be a risk. If the pool was that milky looking, no one would want to get in.
