Get the vector vertical to the direction from camera to object, and multiply it with mouse_force.y or whatever you use to move it:

if(mouse_force.y)
{
vec_set(temp, camera.pan);//get the angle of the camera
temp.pan += 90;//turn it
vec_for_angle(temp, temp);//get the direction of the angle which points aside the camera
vec_add(camera.x, vec_scale(temp, mouse_force_y.));//move the camera depending the mouse_force aside
}

The code isn't tested, maybe, I missed something.