Question

Describe how the wavelets that form in the wavefront apparently vanish behind it?

Answer 1

Here is a demonstration of the difference between group velocity and phase velocity.
Fool around with the upper slider.
Note that when the slider is in the middle, velocities equal, the wave shape moves at the same speed as the group of waves. As you change the ratio of group to phase, the wave shape gets ahead or behind the motion of the group.
http://galileoandeinstein.physics.virginia.edu/more_stuff/Applets/sines/GroupVelocity.html