the only problem with the super high ninja is in vamp form you can't ninjaform so one of the few reasons to have high ninja it is taken away. i'm not sure its worth being a vampire in pvp, if you leech 20% of a 35 pt AI its not that much health, a lesser heal would heal more. I did run vampire in PVP a few times, I did enjoy the no poison thing (as idiot newb mages would spam poison on me), enjoyed the extra MR, the leeches if monsters showed up.. etc... but have been experimenting without it as well. if you have it though i would def keep healing, relying only on leeches will get you in trouble.
I have added ninja (and not stealth or hiding) on my sampire however, I did it when I was a swordsman and would fight Navery, bane guards, or mephitis. Basically anything which hits like a brick truck or summons, then the mirror images are an excellent resource. but when i am not fighting them, i stone it off. i dont feel its a good enough of investment to keep on ALL the time, as I would keep on bushido.
lynk's idea of the shadowstrike is interesting, if anyone raids his spawn he can hide and stealth away even pushing through them in wraith. maybe even gank them one by one as they are fighting the champ. haha. also during the spawn it can kind of act like ghetto Peace in that he would constantly be hiding and getting aggro off himself. i'd be interested to hear more...