After I got too orippled up to climb under cars, I used a hand pump and tubing to get the oil out of my 240Z through the dipstick tube. Seemed to reach all the way to the bottom of the oil pan, but I figured even changing 95% of the oil was better than changing 0% of the oil.
Zahc, vacuum vessels have to be a lot thicker than pressure vessels. With a pressure vesssel, the tensile stresses tend to "spread out" around the walls, but with a vacuum vessel, the outside compressive forces seem to concentrate on one weak spot and the vessel collapses catastrophically.
If you've ever wondered why vacuum cleaner hoses are ribbed, that's why... extra strength to prevent them from collapsing under the "vacuum." Same thing with large vacuum-packed food cans. You'll notice a lot of them are ribbed around the middle of the cans to keep them from collapsing.
I found out about pressure-versus-vacuum vessels once when I had to design a pressure/vacuum chamber for simulating altitude differences. A pressure vessel to handle the design pressure needed a wall thickness of only 1/16"*, but to handle the vacuum, it had to be 3/8" thick. (Consult your "Machinery's Handbook" for details.)
Terry, 230RN
*Actually, it was a lot less than that, but one had to consider handling stresses, etc.