Effectiveness of techniques to improve the performance of rim brakes
You did the right thing by replacing the steel rims. Vintage bikes are nice, but the 70's can keep their steel rims. :-)

As others have said, you should make sure that the entire brake system is properly adjusted. 

Since you're dealing with an old bike, I recommend just replacing the brake cable and casing. This can be done for less than 20 dollars. Before inserting the brake cable smear a thin layer of grease on it by running it past your fingers with a dab of grease.

If your levers are in good shape, slather the interior pivot points and cable anchor with grease. If not, then just replace them. Again, that can be done inexpensively-- and you can get "aero brakes" which route the cable under the handlebar wrap (non-aero brakes are another thing the 70's can keep!).

Finally, there is nothing wrong with single pivot brakes. Yes, dual pivot is marginally better because they tend to stay in place (not rub the rim) and you get a barely noticeable increase in leverage. You absolutely will want to replace your brake pads, however. Modern brake pads are sooo much better. I and many others have had success with the koolstop dual compound brakes.