The basic idea is 1. Pump moist air out of the board while 2. making sure the air entering the board is absolutely dry. I can use a vacuum pump from my work, but I am afraid I will collapse the board if the pumps pulls too much vacuum. It varies from board to board, and also depends a lot on its general health. A big, light, flat-bottomed Mike's Lab, however, will start to deform at only 15 inches of Mercury.
Most boards fall somewhere in between, collapsing sooner the lighter they are.
Don't even think about doing this without a gage. Also, it seems to not speed the drying to leave the pump running all day. Personally, I wouldn't use the board spinning technique because when you drilled a hole at the front of the board no water came out from there, so maybe the foam is quite dry at the ends of the board and it would be a shame to spread the water more in the board. What you have now is a really humid core around the vent area and I think that the technique describe to you by Frank Weston by doing a Vacuum Bag is probably the most adequate I can think of in your case. Instead of going to the dump and finding a washing machine, go and find an old refrigerator.