When I moused over the NPC the default Blizzard tooltip said 'Darnassus'. According to wowhead Cylania <Night Elf Armorer> has a Faction of 'Darnassus'.
Source:
http://www.wowhead.com/?npc=4164
Also, I tried this with a vendor in Dalaran with a faction of 'Kirin Tor' and still got 'Alliance'.
I believe there is a different method I need to find what faction a NPC belongs to.