The more lift created, the stronger the vortices.
Technically, the vortices may be generated earlier in the takeoff roll, since most airplane wings have a positive angle of incidence/attack when on the ground. However, they become MUCH stronger at rotation when the wings start lifting the entire airplane and the airplane starts climbing. During rotation, the wings go from producing enough lift to raise the wingtips, to more than enough lift to carry the airplane, all in 2-3 seconds.