I tried the vanilla scented Christmas trees last summer for our coop. They did seem to help with the flies. The main issue I had with them was that they didn't seem to be effective very long. I don't remember exactly how long they helped...maybe 2-3 weeks? I'll probably give them a go again this summer, only this time I'll stock way up on them before they are all sold out.
Also, I learned that if you have trouble with gnats or no-seeums, a touch of vanilla extract (the real extract, not the artificial) dabbed on your temples will keep them from pestering you. I'm a gardener and try to be out early morning/early evening to avoid the heat, unfortunately that's what the no-seeums do too. I've tried the vanilla extract and it really does work!
P.S. I am a former Kansan, used to live in the NE part around Topeka & Lawrence. You're going to want to do something about flies with the hot summer coming up!