Stretching and getting your heart rate up
I think warm-up really depend on the type of cardio-vascular workout that one chooses.
For example, if I am doing weight-training, I would do a 5 - 10 minute jog to limber up and then some stretching exercises to warm-up the muscles as this is very important to avoid muscle injury.
One the other hand, if it is just cardio, I would start up by doing the lighter routines of the chosen workouts.
For example, before starting on a 5 mile run, I usually concentrate on getting my heart rate in the "zone" before actually embarking on the run.