VDOT was created by the famous American running coach, Jack Daniels. VDOTO2 is a simple, but precise, scientific way to define your current running fitness. By cross referencing a recent race time (5k-marathon) using the VDOT tables, you can determine your current VDOT fitness which is defined by a number (30-85). The higher the VDOT number, the greater your fitness level.
VDOT training allows you to train based on your current level of fitness- based off a recent race or time trial. By using this method your are less likely to overstress your training, thereby reducing your risk of injury, fatigue and improper physiologic training. It becomes scientific instead of random guessing where you should be training. Jack Daniels does recommend training at a particular VDOT value for at least three weeks before moving to a higher one. After the third week, you could then start training at the new training paces if you so desired, but only if the workouts can be completed confidently and repeatable. This means at the end of the workout, you completed each interval at the same pace and felt that you could have done more without a problem. If your effort was too hard, or slowing, then you are training at too high a VDOT level and should adjust your plan. Perhaps even better, if you have a chance to race and establish a new PR, you can reset your training based on the new VDOT value. I would not encourage you to race your way to fitness trying to increase your VDOT value. If you chase this number too hard you WILL wind up injured and out of training for some time! Slow, progressive growth is the key to increase this value.