Most every modern central vac has the option of venting to the outside (much like a clothes dryer).
If you're looking to install one yourself, I suggest doing some on-line research. You'll find a lot of vendors that sell/install central vacs. For the most part, the systems themselves are all pretty similar. The attachments do vary from one to the next, but I was told that there's only a couple manufactures for most vacuum attachments (including portables), and that many are just re-packaged.
I had good luck with a company out east called Galaxie Central Vac Systems. They were very helpful whenever I called, had a good, do-it-yourself booklet that told you how to install, and had some of the best prices I've found. You can call them at 1-800-238-2294, or visit them at: http://www.galaxie-vac.com/default.htm.