Water and pressurized air injector pumps cost here Eur 30, what you get depends on your water temp, 100 mbar absolute is even with warm water. There is no doubt that the more air one gets out of the wood the more oil can get in and the deeper it goes. It depends on what one wants. I have done it both ways and surface warming works quite well on large objects, "vacuum" - actually meaning low pressure in this context - does give a better result on small ones.