Figured it out:
uniform mat4 osg_ViewMatrixInverse;
uniform mat4 osg_ViewMatrix;
void main(){
vec3 camPos=osg_ViewMatrixInverse[3].xyz;
vec4 position_in_view_space = gl_ModelViewMatrix * gl_Vertex;
vec4 position_in_world_space = osg_ViewMatrixInverse * position_in_view_space;
if( camPos.z >1000.0 )
position_in_world_space.z = position_in_world_space.z+700.0;
if( camPos.z >5000.0 )
position_in_world_space.z = position_in_world_space.z+1000.0;
if (camPos.z< 300.0 )
position_in_world_space.z = position_in_world_space.z+200;
position_in_view_space = osg_ViewMatrix * position_in_world_space;
vec4 position_in_object_space = gl_ModelViewMatrixInverse * position_in_view_space;
gl_Position = gl_ModelViewProjectionMatrix * position_in_object_space;
}
One needs to transform gl_Vertex (which is in object space coords) into a world coordinates through view space coordinates (maybe there is direct conversion i dont see) than he can modify them and transform back into object space coordinates.