While Vaylin would almost definitely relate more to the Sith Code, I think that, out of all my Outlanders, she and D’leah would probably relate the most to each other. While Lorna(who isn’t an Outlander, but Bralova’s light-leaning Sith ally, and someone I should make a post about) was born into slavery and only got free by defending an attacker with Force lightning, thus displaying her Force powers, D’leah was targeted by Vitiate because Tol Braga was stupid enough to think the Emperor could be redeemed. There’s also the dialogue when he possesses Kira after defeating Darth Angral. “I see futures in you. Futures I will not allow. Your future ends here.” Vitiate was terrified of D’leah because he knew, one way or another, she was eventually going to end him. That’s one of the reasons he made her and the other members of the Jedi strike team his puppets. Of course, he didn’t think she’d break free with Orgus’ help.
D’leah didn’t have nearly the mental breakdown Vaylin did, but she does have PTSD, depression, as well as recurring nightmares. She also started drinking in the KOTFE era, but thankfully Koth and Lana got her to stop that. The stress was a lot for her: seeing your BFF getting possessed, then getting turned into a puppet, then killing only what turned out to be your biggest nemesis’ Voice, having him resurrected by a batshit legendary Jedi turned Sith turned Jedi, and then having the motherfucker make your brain his new home. She knew Jedi aren’t supposed to be afraid, but this is Vitiate we’re talking about, not even her former rival, Xenli(the Wrath in the Ebon Hawk legacy, who’s light-leaning anyways).
Probably what does help her is the support of her family and friends. Vaylin sees this and probably gets jealous at first. But when she sees the Jedi refuses to use her trigger phrase, she gets curious about how D’leah and her mother, Ursulina, aren’t typical, emotionless, stoic Jedi.