Anyway, the chassis configurations of vehicles seem pretty clear by their own structure. APCs, Hurricane, TA-6500 and EDD have certainly closed chassis, and I would include Strike Skimmer and Mekas too. All others vehicles have open chassis.
See, i would define the chassis type on very different reasons....
For me it makes by the description only sense, that the only "open" chassis are the ones that are not closed around the structure itself. I would say that anything that hinders movement and flexibility of driver and vehicle leads to a "hindered" or better limited facing.
I would say the bikes (Pegasi, Divine Wrath) are open in structure ( the driver can see what comes and is pretty flexible).
Near to that are the Necromowers and Fenris Bikes, basically framework with pitch and paper; and the Fenris is even allowed to attack in melee.
Orcas are closed in my opinion.
battlewalker would be argueable as would Great Greys and Purple sharks be; just compare new and old models.....
Thatīs why I think you should find a solution within your group. The rules donīt cover this part I think.
--> Or you can just adapt the rule that AV-rating is deduced anytime you draw LOS from the back of the vehicle, e.g. a 90° arc measured from the middle of the back of the vehicle.
MAkes life easier in this case than debating about the rules that arenīt specific in this part ;-)