Shoot the ramp with the other ball, nudge the ball in the plunger lane to trigger the plunger switch, vortex multiplier increases by 1X.
If the ball on the ramp looks like it's heading into the plunger lane as well, fire off a skill shot before that 2nd ball lands in the plunger lane.
If it doesn't, you get the free vortex bump without having to do anything. Shoot the ramp again, repeat process.
I'll usually start plunging skill shots at 600,000 to try and cash in at 600,700,800,900 and the million. Once it rolls over, go back to bumping it up to get through the lower values easily.