Vojvodina was not part of Serbia then. Arreas abot Morava was inhabited by Slavs, but it was changing overlord constantly. Serbs, Byzantines, Bulgarians, Hungarians. In time of battle of Kosovo it was Serbian, and Center of Serbia, Moravska Srbija.
This was Lazar's domain:
Here you could see realms of other lords (Konstantin and Dejan Dragaš, Dušan's nephews, Vuk Branković and Đurađ Balšić, Lazar's sons-in-law, and finaly Marko Kraljević de-jure King):
Well to correct map, Smederevo was built much latter, and Beograd was Hungarian.
Concerning Vojvodina, Srem was inhabited by Serbs, second Serbina archbishop, Arsenije I was born there.