Building Vertex Group Particle System in Blender

First of all we need to take a plane and sub-divide it in several time.

Then we add particle system to it and change emitter to hair. under render option

we change it object type and add a reference object which we create in different layer.

We also reduce the amount of hair 1000 to 100.

Selecting the plane under under object data tab we create a vertex group and name it custom_place_particle.

Then selecting the plane in weight paint mode we paint some custom place to add our

object.

Finally in particle system under vertex group we add that custom_place_particle
and bingo.