Edited to say, this might not be When Vampires Attack itself, but something else interacting with it.

I tried to edit the title, but I guess that's not possible! I don't want to give the mod or it's author a black eye here!!!

So I hit level 9 tonight, and had my first vampire attack in Whiterun. As expected, the innocents all fled, except for the courier who was stuck in the middle of it all, he knelt down and covered his head. Unfortunately, he is now stuck doing that over and over.

I tried hitting him with unrelenting force, well the first word of it anyway! I also tried moving the dead Vampire bodies away from him, talking to him, fast traveling away and back, and sleeping for 9 hours. He's still standing there in front of Warmaidens, periodically kneeling down and covering his head.

Any ideas of how to clear this, other than loading the last save? It's a couple of hours old, so I'd like to keep going if I can!

Are you on PC? If so, just open the console, click on him, type "disable", hit enter, then type "enable" and hit enter again. That should reset his AI so you can talk to him.

You can try leaving the cell for at least 10 days. That might be enough to kick-start him. (In other words, go to another city for that long - somewhere NOT close to Whiterun.)

That did the trick, thanks!

Glad to hear it, happy adventuring!