I did a lot of research in this subject several years ago. Polar has the paper it published regarding this available on line. The monitor is within 2% correct. I've had my VO2max tested and my polar watch was within 2 values of my actual test.
One day, I decided to test it. I tested it in the morning, after eating, after working out, after eating, after working out, while talking to someone, while driving, etc. It gave me 74 or 75 every single time for 12 tests total. This is what my most recent actual test came out to.
Now, am I a 4:07 miler. 14:13 5,000m runner or a 1:05 half marathoner? No. VDOT works well, but you have take other factors in to consideration: muscular endurance and the most important: economy.
I know people don't like to hear this, but you can improve your economy. The article written by John Kellogg was good, but not perfect. If you want perfect, read The Pose Method of Running book. Before knocking it, read it. If you think that this model of running is flawed, then you are not understanding it fully.
With improved mechanics, I've been able to train more, injury free and improve upon my times. Six years ago, my VO2max was tested in a lab at 69. My mile time was 4:54. With improved mechanics and a VO2max still at 69, my mile time improved to 4:25. With even more training, my VO2max is now 74, but that's only possible due to good mechanics, which are outlined very well by Pose, which have allowed me to recover faster, run injury free, more relaxed and Faster. I still have work to go and I hope to get closer to perfect form and take full advantage of my VO2max.