Was lucky enough to get this fairly early. Been getting a lot of questions as to how I actually did it, so I thought I'd post my take on it here. Not so much of a guide as a play-by-play, but maybe it'll help someone out.
I started out alternating between Advancing Time, and Fast Traveling between the 2 maps. After about an hour of that with nothing to show for it, I decided to try clearing out (Dispatching) a few Captains manually to make some room. The first map (Udun) was pretty much 100% Sauron controlled, but I had about 40% of Captains on the second map (Sea of Nurnen) Dominated. So I Dispatched 3 Captains there (via a Dominated Duel Mission) and tried again and immediately got a Vendetta to spawn about 1,000 meters east of the central Forge Tower (on a small outcrop with a zip-line pointing south-south-east). I ran directly to it (no Fast Travel) and initiated it. Once I uncovered the Captain's location (on the cliffs to the east) I took him out as quickly as possible (no checking for or acquiring intel) ignoring all Bonus Objectives. My strategy was basically stealth first, then arrows. Failing that, thin the herd using arrows/fire and then to go to town on the Captain, all the while trying to avoid giving him a chance to Flee (the last thing I wanted to do was waste an opportunity or break the streak).
Doing this I was able to knockout the first 4 Vendettas with back to back to back to back to back time advances (from the central Forge Tower). All in the same location (spawn point and Captain location). All low level Captains (this is my best proof that manually clearing out Captains is the key, as up until that point this map had predominantly consisted of high level/Elite Captains). All Avenging the same player (someone I have been friends with for a long time i.e. both following each other - who played the game for the full K, but interestingly enough hadn't played in at least 5 months). This would seem to indicate that the game does have somewhat of a 'memory' and that it might not be necessary to have a friend who is
actively playing the game in order to get the challenge/achievement, just someone who has at least played it in the past (and died, obviously). It's also worth mentioning that I did try adding additional players (see
Lost but not forgotten - Vendetta buddy) and that I also have one friend who is still actively playing the game, but that I didn't get a single bite on any of them.
The fifth one showed up after two more time advances (same player, different location). The whole thing took about 2 hours from start to finish. Hope this helps :)
Side note: What the heck was Monolith thinking making this the final challenge?!