We have a lot of tansy here; that's supposed to deter flies. I don't know, though... it's considered a noxious weed and the feds are constantly spraying it in the national forest land all around us.
They send in a bunch of idiots who apparently can't tell aspen and birch from tansy.
It doesn't kill tansy but it's awfully hard on aspen and birch. I cut a bunch of tansy every year from where it tries to creep onto our place. Last year I tried soaking it and using it as an insect spray on my garden but it didn't work. I think it made a fairly good fertilizer, though
I would not use vinegar on a metal tray; it will even rust galvanized metal. I don't have rabbits yet, but I learned this with chicken feeders from feeding fermented feed. They
love
it, but it's a good way to ruin expensive feeders. Now I use those recycled rubber pans--wonderful stuff!
You could sprinkle into the pan some chopped straw or other (chopped if it's broad leaved) dry plant matter that would soak up some of the urine. Or you could put the bedding onto the cage floor, even a couple of inches thick. You would have to scoop it out every several days if it doesn't sift down into the pan, but deep bedding does a great job keeping the flies down for my chickens. In fact, I've never seen much for flies in their coops. I use pine petals that the squirrels gather up for me, and other soft forest floor debris. If you want to use fallen broad leaves, you'll need to chop it. A lawn mower works well, but point it toward a bare spot like a driveway, so you can easily rake it up.