You mean like this?
temp.x=0;
temp.y=0;
temp.z=200;
vec_for_screen (temp, camera);

It doesn't do anything right now, is there something else I forgot?

EDIT::
Nevermind, it works now. I just have some trouble placing it in the right spot.

EDIT::
I have another question. The particles keep going through the ground, what can I do to change this?