I'm trying to achieve a vacuum by sending air through the green tube, (in to out) sucking in air from the in/out blue tube. pulling the piston back to the end
Are you trying to create a sort of Venturi effect? http://en.wikipedia.org/wiki/Venturi_effect
Venturis work well for low pressures, but you'd need a lot of airflow to drop the pressure very much.
And where does a piston fit into this? I don't see anything resembling a piston?
It's entirely possible, but the shape of the tubes affects the results. I have one of these that I bought for making a suction filter from tap water flow. I believe you need a nozzel so the air flow is introduced at smaller diameter than the tube it is blowing past. A tiny bit of googling would probably clear up this point.