Hi there,
I have a diagnosed BPD and an undiagnosed NPD in my life, and I have been surprised at how their behaviors seem to overlap: the delusional, extreme beliefs, the incessant need for attention, the entitlement, the demandingness, the manipulations, the lack of empathy for others, the volatile relationships, the million excuses for breaking promises, blaming others for everything and twisting facts to justify hurting others. I'd say that, on top of all that, is the ability to "pull themselves together" when they want to, as well as their apparent willingness to hurt people close to them.
I'll note that I understand that many pwNPD can appear to lead functional lives, but the pwuNPD in my family has a dysfuntional lifestyle. In fact, both the pwBPD and pwNPD rely on others to over-function for them in life.
Nevertheless, I see two main differences in them. The first is the underlying mindset. I think the pwBPD is beset by overwhelmingly negative thinking. Her default assumption is that she's incapable, unloveable, messed up, unworthy, ugly. She's insecure and plagued by fear, worry, anxiety and shame. She excoriates herself over every little thing. Since that's too disturbing to live with every day, she flipts the script and focuses outside herself, convincing herself that she's mistreated, aggrieved, downtrodden, unlucky, abused by the world. That leads to projection of her ill thoughts onto others, and to blaming others for messing up her life. The negative mindset morphs into a victim mindset, which keeps her stuck in a rut, because she refuses to take responsibility for her life and behavior. Instead, she lashes out (or retreats in avoidance) as she blames others for all her problems. She believes she's justified in hurting others, in a misguided attempt to let them know the extent of her pain and/or to punish them for perceived wrongs.
In contrast, I think the NPD is beset by a "delusional," overly positive view of himself. He thinks he's special, important, privileged, unique. He is a spotlight hog and thinks that other people should not only praise him, but serve him like a king. He is demanding of others in terms of following rules, and yet he is exempt from any rules because of his special, exceptional status. His interests and needs always come first, and he doesn't consider other people's feelings, not even those of his own children. He's always the best, always right, always the most important. When he's getting attention, admiration and service, he can be charming and engaging. But when the attention fades, he can turn in an instant and be cruel. Yet his retributions seem more calculating than those of pwBPD, who are ruled by emotions. I think the pwNPD is more controlled--seemingly intellectual and calculating--when plotting retribution. It's as if he enjoys seeing others suffer because of their insubordination and inferiority.
And that leads to a second critical difference. I think the pwBPD might be capable of admitting that her life has become unbearable, and that she needs some therapeutic help. She might feel so miserable and desperate, that she'll seek out therapy and actually give it a chance, provided that she has no other option but to work on herself. If she's determined, she has a chance of turning her life around with therapy. But I suspect the pwNPD would never admit to any need for therapy. I think he believes that he's perfect, that therapists are stupid, and that his brain and body are special anyway, immune to conventional treatments.
As an aside, I think it has helped me to think of my pwBPD as someone who is emotionally handicapped. I've estimated that her emotional "age" is around 70% of her chronological age. So at 21, she was operating like a 15 year old at best. Think teenage relationship crises, over-reaction to minor setbacks, impulsivity, procrastination, difficulty with accountability, reliance on parents to manage her life, peer group obsession. As she has gotten older, I think she has matured somewhat, but in her late 20s, she's still somewhat "behind." She seems completely overwhelmed by a relatively easy, entry-level job (basically, office reception with scheduling duties). She can't get along with her roommates for very long. She's flaky and bails on commitments. She holds grudges forever and can't apologize. She takes everything too personally, such as feedback from her boss. She still expects other adults to over-function for her, though she has made some progress here. Anyway, my observation is that while pwBPD seem to be emotionally "immature," I don't think the analogy works very well with NPD. In other words, I've observed that pwBPD can grow/mature/change, whereas NPD, not so much.
Just my two cents based on anecdotal evidence.
