The tube has dry rotted I was able to shorten it to make the system work but I can't find a part # for a replacement.

What type of line is it? Usually a good hydraulic shop in your area can make tubing or hoses of many different kinds. You might check there. Its usually not too expensive and you may end up with something better.

It is a very small diameter about 8" long vacuum hose. It is on a Ruud horizontal furnace.

If you can't find it somewhere near you or online. Take it to a hydraulic shop where they make hoses. In most cases they can duplicate a hose or tube in a few minutes.

Take old hose with you... even most auto parts places can help find one

Your speaking of the rubber hose between the inducer and pressure switch yes? IF so, you can simply go to an auto parts store(as wmproop already suggested) and ask for the same inside diameter hose, or better yet, go to any plumbing and heating store(take the old hose with you) and they should be able to give what you need. If your looking for a factory replacement(not necessary), you can get the model and serial of the furnace and take it in to a plumbing and heating shop and have them order the part.