OMG I was NOT expecting that answer at all!!

When you’re considering Vaughn’s motivations, you have to keep in mind that he’s a genuine psychopath. He has no empathy. He is incapable of thinking from another person’s perspective. To him, he is the only person who is actually real, and everyone else is a bit player on his stage - projections of his own desires, rather than people in their own right.

What Vaughn thinks he wants is a romantic relationship with someone who knows what he’s really like. But what he actually wants is the fantasy he’s created of that relationship, in which he does whatever the hell he likes and his partner lovingly and happily caters to his every need and desire. What he wants is impossible, because it doesn’t take into account that the other person in the relationship would be a real human being instead of a projection.

The closest Vaughn can come to that fantasy is with a person like Desi, who already had multiple risk factors in place that made him more susceptible to manipulation and emotional abuse. What Vaughn has done to Desi would never have worked on Allison, and he knows that.

Not all of these decisions are operating on a conscious level; some of them are more the subconscious instincts of a predator. Desi is a very strong person in general, but his specific weaknesses were exactly the ones most useful for Vaughn’s purposes. Vaughn’s psychopathic brain interpreted this as Desi being the perfect partner for him, whereas from an external perspective, it’d be more accurate to say that Desi was the perfect target for him.