Small World: Do Orcs Get Race Bonus When Conquering Abandoned Territories?
You have it backwards.  Orcs do not gain a bonus for conquering unoccupied territories.  From the Rule Book, emphasis mine:

Orcs
Each not empty Region your Orcs conquered this turn is worth 1 bonus Victory coin, at the end of your turn.

Vacating a territory would mean that it was unoccupied when conquered, and therefore the orcs would not get a bonus.