Champions of the Loa (Which Vol'Jin was) are essentially as immortal as the Loa want them to be. The flexibility Blizzard gave themselves by writing in the Loa make death for Troll characters as meaningless as Blizzard demands. Vol'Jin could be dead for good because "da Loa say so" or he could come back as a powerful spiritual entity to rally the Horde or Zandalari.
Vol'Jin does have a pact with Bwonsamdi after all, who is as powerful as Blizzard wants him to be. I'm going to guess Vol'jin isn't dead dead, and judging by the tears of Horde players complaining Vol'jin died before he even got interesting Blizzard would probably bring him back soon, if not in a patch in the next expansion.