First let me say that vacuum pots not only produce some of the best coffee you are likely to ever be fortunate enough to drink but they also look really cool while they are producing coffee.
How do vacuum pots work? The buildup of steam in the lower bowl forces the water up into the funnel, where it mixes with the ground coffee. A quick stir may be needed to wet the grounds into the water. A small amount of water is left behind in the lower bowl. This keeps the steam coming and the temperature constant. Brewing continues for 1-3 minutes. Then the siphon is taken off the heat. With no more steam being produced, a vacuum forms in the bowl, which sucks the brewed coffee down through the filter.
As a note if you have a vacuum pot that uses a spirit lamp to heat the water you will probably want to warm the water to a near boil before adding it to the vacuum pot. This will make the process much quicker since a spirit lamp can take quite a while to boil water.
Most people feel that you should allow the water to come to a boil or near boil in the lower chamber before putting the upper chamber on your vacuum pot. This will help reduce the chances of cool water getting pushed into the upper chamber before it is ready.
The Cory brand vacuum pots use an interesting glass filter. It fills the hole that coffee is pulled back through via vacuum after the heat is removed. I honestly do not know exactly how it works but it is able to keep sediment almost completely out when using finely ground coffee. This is the most efficient method of keeping sediment out of the final coffee that I have ever seen with the exception of paper filters. Unlike paper the Cory filter doesn't trap any of the oils in the coffee. I would highly recommend trying to find one of these filters if you have a vacuum pot that can accommodate using the glass rod. Unfortunately Cory is not longer making the part but there seem to be a number of them available online. I have used (and continue to use) a Cory rod in all of the Vacuum pots that I have owned. I feel it makes a real difference. I have not had a pot yet that the Cory rod won't work with but I'm a little reluctant to say they will work with any and all vacuum pots for fear of being proven wrong.
You should take care when serving and drinking coffee made in a vacuum pot it will be much hotter than with an automatic drip pot.
I'm told Vacuum pots also make good tea.
A few vacuum pot brands include:
Cona (the original) in England
Hario in Japan
Hellem in France
Yama Glass in Taiwan
Cory, Sunbeam, Silex, Kent, Remington Electric and Westinghouse have all made vacuum pots in the past.
For more information and great pictures visit the vacpot page at oldcoffeeroasters.com.
A couple of companies are now offering electric automatic vacuum pots. I have tried two (I think the only two) brands and they are both OK but not as good as a manual. If you really want the convenience they are probably an OK compromise but remember there is a compromise and you will not get as good a product as you get from a regular stovetop vacuum.
As with other types of coffee makers vacuum pots work best if matched to their expected capacity so if you usually drink a small amount of coffee look for a vacuum pot with a small capacity. If you are planning to hold onto your coffee you will probably also want to invest in a vacuum bottle to keep the coffee warm.