The vent plug/rod is there to dissolve any air bubbles that may be in the oil. It floats inside the tube so it's normal to rattle around inside. Now that it's broke your best bet is to take it to your local oil heating co. and have them match it up with one of the same length. They might have a junk pile you can pick through...Its very important to find one of the same length or you'll have to have a service tech set up the burner. The part you broke is called the Drawer Assembly. If you measure the length I might have one I can send you. What model burner?