All exercise is beneficial.
however, to improve cardiovascular fitness, you must maintain a 'training' heart rate for 15 minutes or more continuously. 'Traning' would be about 75-85% of your maximum heart rate. quickstep and viennese waltz will put your heart rate up very quickly and it's not possible to maintain this for long enough. there is also general and specific fitness. even a very fit person could get 'winded' when they start dancing because they haven't developed the specific fitness.
as i said, all exercise is beneficial, but in my view, the gym is the best way to get fit for dancing.
Running at 8 minutes-per-mile pace will burn about 800 calories per hour.