Hi,
I have one a vacuum pumps, but, to be fair it is one of the cheapest on eBay. I did not have that much luck with it as the pipe must have curled up in the oil pan as it only recovered about 1 litre of oil. The quantity of oil recovered should be around 3.6 litres. The theory of it is sound as for loads of garages the vacuum pump is the default method of oil removal.
Ben