Switching to sand in my run helped a lot. Moisture seems to be the key. I used hay prior to switching to sand. The hay was wonderful until we had some heavy rain and the bedding got wet. Then I would have an explosion of flies. My inner coop is rainproof and I use a fairly deep layer of horse bedding so its basically a desert environment in there. It was the outside areas that were the problem for me. I'd dig up some wet hay and find a host of maggots. There was virtually no poop in the hay either since I would scoop it out on a regular basis. The flies came for the hay more than anything as far as I could tell. I've switched to sand and not only is my run super easy to clean my fly problem has all but vanished.
Another mistake I've made was getting a "bait" type trap for the flies. For one thing it stunk soooo bad....it smelled like rotting garbage. I had to move it outside of the coop and by the end of day 3 I had around 400 dead flies STUFFED in the thing. A small fraction of those were residents of my small coop. The rest must have come from miles only to die in my trap. I must admit there was a bit of satisfaction knowing I had killed so many flies but the trap was counter productive since it attracted more flies than it killed.
I've also tried sticky traps and they seem to accumulate more feathers than flies...though they didn't attract more flies than they killed and they were extremely affordable. So, it was a lateral move.
I have not tried the vanilla, but it sounds like a pleasant smelling plan at the very least.