Here's the deal with roof racks (at least, from
my point of view):
generally speaking, for the sake of looks and aerodynamics (not to mention ease of access), you should keep them as close to the roof as reasonably possible ... bike rails, ski racks, kayak carriers and the like can easily be placed on either side of your roof vent to allow it to function properly, and canoes (or similar open-top boats) can be placed up-side-down over it. However, if you plan to ever put a basket (like mine) or a pod of some kind up there, you'll either have to set the assembly to adequately clear the vent (taller towers, different placement of the towers, etc.) or come up with a system to lift said basket or pod high enough to open your vent (again, like I did). How far you need to open your vent depends on where & when you do your camping and how much air flow you need/desire ... I can only open my vent a couple of inches, but I've found that it is plenty for my needs (no, I don't have a fan in my vent, and have found that I don't need one, but if I did, a slow speed setting would be all that was necessary and would be fine with my 2" vent opening).
Mounting the racks higher (ostensibly to clear the open roof vent) works against you in at least a few of ways: it paces the load higher and therefore more difficult to access, not to mention roll-center/center-of-gravity (even on mine, I have to stand on my fenders to access and secure some of the items in the basket) ... it will more adversely effect the aerodynamics and therefore your fuel economy (and I can testify to that!) ... and it just plain looks weird with all that stuff sitting so high above the roof of your trailer.
IMHO, roof racks are a mixed bag of blessings & curses! I like the pure simplicity of minimalist camping and the clean lines of the basic teardrop, but I sometimes liked to take a bicycle along for campground cruising, so at first I mounted a simple bike rack on the tongue of the trailer ... then I started carrying my ice chest up there and needed to move the bike to the roof of the trailer ... then my back issues deleted the bike thing and I was looking at a kayak ... then I added a more-often-than-not camping partner which meant more "stuff" needed to be packed, so the basket got added ... and on & on it goes ...

...! My mileage when towing has gone from being barely affected to as much as 6 mpg less (even more when I add the racks & associated "stuff" to the tow vehicle ...

...!). I used to be able to just pull in to the campsite and pull out the camp-chair and put up the umbrella, but now it can take quite a while to set up camp (and even longer to pack it all up)! In the end, it really all depends on your particular needs and camping style and sense of style & aesthetics. However you approach it, just think it through thoroughly and carefully, and choose the best product(s) to suit your needs/desires ... after all is said & done, you won't regret it ...

...!
Happy Camping, and as always ...
CHEERS!