Before the Thydian Civil War, Vetheriss was a smaller Arling. The Arle of Vetheriss claims the lands claimed from Felkhath were annexed peacefully, and the people of the far nothern reaches of the Arling had asked to be taken in until I, "see reason" and side with the Capitolist Faction in the war.
These lands are still Felkhath and Norrasjo. We are their custodians until their Arles see reason and join us, or formally declare war against you, the rightful heir. It is not that hard to see.