I heartily disagree with that, more than I can easily explain. "A different history, cultural context, and ethos" and yet "a point of similarity" to put it your own words is enough to make them extremely similar, or even representative of an archetype? The thing the Blood Knights and Vindicators truly share, in the context of WoW, was a common enemy: the Burning Legion and specifically Kil'jaeden. It is that shared animus that gives them common cause at the close of TBC, but aside from that you'd be hard-pressed to find two groups with less in common with one another. Not to mention that the Blood Knight's history of imprisoning and forcibly siphoning the Light away from one of the Naaru is likely enough to make your standard Vindicator be sick to the stomach. As for the roles they serve in their culture, that's seems like a pretty substantial leap. We know the Vindicators are the protectors of the Draenei, and that their elite even serve as the honor-guard for Velen himself. The Blood Knights are part of the Thalassian military but don't seem to have anywhere near the acclaim as the Vindicators among the Draenei do - in point of fact their legacy is streaked with distrust, and many Sindorei seem them as haughty or aloof. The deeper you peer into the specifics of all these things, the more and more differences and distinctions you find, and the argument for shared similarity becomes harder and harder to justify.