Do you think the vent would hold that much weight? it would probably break off the first time I hit a bump. OTOH, I heard that you're not supposed to mount them in the car for navigation, but I think they mean you shouldn't put them on top of the dash because it'll block your view of the road.
So you're left with a window mount with a long arm to hang over the edge and it probably blocks the vents, which is OK with me- I have windows that go down
I think one of these would be great with a windshield suction cup mount. Just position it so the tablet rests on the dash. Shouldn't block the view of the road that way and would be pretty sturdy too.