I was wondering if there were descriptions of Modern Psychological Narcissism in Buddhist works (as opposed to vanity in the Greek myth)? Is it closely tied to notions of Ego?
In buddhism, what suffering ISN'T a problematic tie to notions of ego?
Joking aside, the personality disorder constructs and buddhism have their own traditions of thoughts, and as as such one can find both similarities and differences in comparison. Your question is interesting, but also run the risk of confirmation bias, as long as one only look for buddhist concepts that fits with mental disorder diagnostics. I believe one may also find diverging concepts in comparing buddhism and personality disorders.
However, since your question actually pertains similarities, some things worth noting is the idea of unwholesome mental factors (akusala cetasikas) in buddhism.
Specific for the etiology of NPD (on the top of my head) is pretty much all of the unwholesome mental factors, with the possible exception of uddhacca (restlessness), kukkucca (regret), thīna (sloth) middha (torpor), or vicikicchā (doubt) which aren't necessary criterias for a NPD diagnosis per se.
Regarding BPD, i'd actually argue that the emotional shifts are only erratic or extreme for a layperson. I also believe that the dichotomous thinking associated with BPD is a bit of a overgeneralization, even though it can be prevalent among those diagnosed. I write this because people diagnosed with BPD run the risk of being burdened with A LOT of unfair misconceptions.
That aside, from a buddhist perspective i'd argue that akusala cetasikas that (arguably) fits with BPD are: moha (delusion), anottappa (disregard for consequence), lobha (greed), uddhacca (restlessness), ditthi (wrong view), or dosa (hatred). Again, these are in no way reserved for a BPD diagnosis specifically.
Looking at the buddhist idea of skandhas, one may argue that the emotional proclivites in BPD relies heavily on constitution (rupa), in conjunction with an unhealthy amount of negative life experiences. However, as far as i understand, buddhism doesn't rely too much on the impact of memories, and seeing past trauma and/or abuse as karma is rather cynical in my opinion.
On can also look at any form of mental disorder as matter of upadana or clinging. As we lose flexibility in dealing with life, mental illness is not seldom the result.
Are there teachings of Buddhism that could be especially helpful for such people?
A little buddhism would likely help a lot of people to improved well being, diagnosed or not.
Fact is that one of the evidence based treatments for people diagnosed with BPD (dialectical behavior therapy, or DBT) is influenced by zen buddhism, and one of the main figures - Marsha Linehan - claims to be a practicing zen buddhist as far as i understand. The treatment relies heavily on mindfulness practice, but there are also other ingredients involved in the treatment, some of which may be contrary to buddhist thought (the idea of emotional validation, or certain aspects of the interpersonal skills training for instance).
A lot more can be said about this, so consider my answer a scratch on the surface. Read it with a grain of salt.