it is not what you ride but how you ride. I have been on a few vintage rides and they ride much slower than I do. some fellow Cabers and I met up for the Niles Canyon ride. fun times and good people. I think it is 6 miles up and 6 back, I went up with them then back to the start alone, then back about half way again and turned around... probably 20 miles VS 12 for the original ride
when I ride vintage I ride like a roadie, fast. get the heart rate up. many times I do "intervals " where you go fast and get max heart rate then coast, then repeat. I can get a good workout in a short distance and minimal time. sometimes I do "hot laps" just around the block for 10 minutes. get the heart rate up, get that burn in my legs.
it is not better to ride a 40 pound bike unless your goal is to build muscle rather than cardiovascular fitness. I feel a burn in my legs on my Phantom I do not get with a road bike.
with all that being said I ride my vintage bikes much more than my road bikes and Mountain bike.
another advantage for a good road bike is the 50MPH downhills I used to do when I was young and fearless.