Summary
If your vehicle has a vent on the roof that you cannot un-attach, you will generally still be able to install a TentBox.
Installing away from the vent
Roof vents usually appear on larger vehicles (such as vans), where the total blueprint of the roof is larger than the TentBox model anyway.
You may be able to install the TentBox without obstructing the vent if your roof bars can be mounted at a different location on the roof.
Installing above the vent
If your roof isn't large enough for this, it is usually still possible to install a TentBox over the top of an air vent, depending on the size of the vent.
The TentBox will sit on crossbars that are raised from the roof itself, so it may be worth searching for cross bars that give a reasonable amount of extra height. Crossbars vary in height, they're usually between 5-10cm.
If you do go with this method, the air vent will usually still be functional, as they normally draw air from the sides, rather than the top.