My experience has shown that scanning your blueprints, then inserting them into a drawing and tracing them with your own standard layers is the best bet. I tried using a CAD Drafting service that scanned a drawing and then converted the scan to a vector drawing and got some bad results. For example, a dashed line will be converted into many small lines that do not necessarily align. Your vectorized drawing will have small bits and pieces from some of the spots and smudges in the background.
I have been using "JPG" scan files and then inserting them and scaling them to match maps for a conversion that we are currently working on. By tracing them, we can make sure that each of the entities is drawn on the correct layer and we can remove irregularities from the old maps.
We have hired local students to do the work, but like it was stated above, there is no perfect automatic way to do this. If you do not need the drawings constantly. Store them with the tif (or scan)attached and when they are needed, trace them to your standards. That way you are not converting them just to convert them, and you can still run a print if needed. My 2 cents worth.
