Update vertex fragment from a texture

Hi, I’m looking for a way to update vertex positions within a vertex shader, passing from Processing a texture (sampler2D in GLSL) taken as an output from a fragment shader, which contains new position’s coordinates (x,y) of the current vertex.

In pseudocode you can see it as this process I guess:

I need that in order to create a particle system that keeps track of the particle positions by modifying the vertex position.

If you need a better explanation or any info I’ll try my best of course, let me know.
Thanks in advance!