If sternal debridement is performed on the sternum do they have to leave it open and do you have to wear a wound vac?

Mor than one way: You are describing one method of treatment there is also a method called one step debridement with muscle flap reconstruction. At least one study found this to be safer.

Many options: If the sternum was debrided due to infection, then yes, it has to be left open to heal from the bottom up. Using suction therapy (vac is just one brand) is an excellent way to help large/deep open wounds heal but the infection must be treated first by removing the infected, non-healing tissue and then antibiotics directed specifically at the bacteria that caused the problems. Wound consult helps.