I always start with cleaners such as Dawn detergent, Murphy's Oil Soap, and then household ammonia washes to remove crud and saponify old gun and skin oils. Ammonia should be thoroughly rinsed and neutralized. Wood should not be given prolonged soaks in ammonia solutions. Then I proceed to heat and absorbent materials such as kitty litter, following up with solvents if necessary. The vacuum sealing and placing the stock in a warm area sounds interesting, but I think it would work better if you wrapped the wood in paper towels or other absorbent material before placing it under vacuum. Simply placing the wood in a vacuum bag doesn't give the oil anywhere to go.
Some here seem to think that prolonged repeated soakings in harsh organic solvent will not do any damage to the cellulose and lignin structure of wood. I think that is ridiculous, and prefer to minimize the exposure to harsher solvents such as acetone or lacquer thinner. You need to strike a balance between removing gun oils that will destroy the wood over time, and removing the bulk of those oils. For that reason, I try to get most of the oil out by using less destructive methods, and minimize the use of organic solvents. Here's a paper on the effects of various cleaning methods on archeological wood.
http://www.ijcs.uaic.ro/public/IJCS-13-16-Hamed.pdfI don't really think gravity will have all that much effect on oil absorbed into wood. Sure, some may drip out of a heavily oiled stock head when heated, but heating will bring oil to the surface no matter what position the stock is in.
