A big question wrote:
Are the shoes only useful for marathon?
Why they don't produce any result on track?
Track spikes already have some stiffness built in with a plastic spike plate. Most shoe companies run this the length of the sole to get more of a spring effect. The spikes give traction that the racing flat cannot. The vaporfly increase in spring effect won't make up for the loss of grip against a shoe that already has a spring effect and is half the weight.
The shoes for the truly elite are probably tuned to exactly their body weight, cadence, flexion, and power. The normal production shoe is tuned to 180 steps per minute and some nominal weight for size of the shoe. This is probably a weight a bit higher than an elite athlete, but lower than the average 1st world runner's weight. So only a fairly light runner gets the maximum benefit.
For those who say "no benefit" let's think back on a few technological advances in track. Were dirt / cinder tracks better than asphalt? How about asphalt vs. rubber? Rubber vs Mondo? Tight turns vs sweeping turns? Did these advances result in faster times? Were flat rubber soles then gum rubber shoes better than blown foam? Did these shoe advances allow runners to train more / harder and therefore get faster? And a directly relevant example: the blade runner. Did his blade calfs / feet allow him to run without the fatigue that a runner's calfs / feet would experience. Could the blades have made him cover more ground (greater spring effect) with each stride?
The answer to all of those technology advances is that they made runners faster, without question. Why then do we believe that a change in shoe technology today would not do the same? The benefit may be small, but a little is often enough. The same is true for PEDs. Combined, a small benefit becomes overwhelming.