Even though Vahan meets these two ladies in different places and times in the novel, we're grouping them together because they bring out the same qualities in him. If you think about it, these chicks have a lot in common. They (1) are used by Turkish men for their bodies; (2) bond with Vahan over horrific experiences; and (3) leave Vahan questioning everything he knows. Let's start with the first one.
When Seranoush first come to the stables, Vahan notices, "she looked dazed, the way my mother had looked after Diran and Tavel were shot" (20.13). He's not sure why she's so distant, and at first, he thinks she's a "gift" from Selim for him to hang out with, a buddy to keep him company during his lonely days. Pretty quickly though, he sees firsthand how the soldiers treat her (they rape her) and understands what is worse than death. It's the same with Seta—she's there for the German consul, to use as he wishes and discard when he's through with her.
Both ladies bond with Vahan a great deal in the short time he knows them. This is partly because they don't have other friends or people to talk to, but also has to do with the fact that all of them have been through dark times that only they can understand. Vahan might only know Seranoush and Seta for a couple weeks each, but as he tells us of Seta, "hours turned to minutes when I was with her, and every day when we had to leave each other, it always seemed too soon" (28.1). He can't get enough of her—of having company and the safety of friendship.
Needless to say, Vahan is super down in the dumps when both Seranoush and Seta die. He's come to care for them and can't understand why these girls had to die. It's unfair and traumatizing for our main man, and each time it happens, we see him struggle to make sense of such intense unfairness in the world.