You don't need to center a vent. The mounting needs to be reasonably flat, not perfectly flat. Most RVs now have a curved roof, and mount vents to the side of center.
Looking "too weird" would be a matter of your own taste, but if there are any passenger van fashion police, they might not approve of any roof bubbles at all. A vent in plywood replacing a window would certainly look more weird than on top.
The. vent will mount to exterior skin, a trim piece covers the open in headliner and whatever is between. The trim pieces come in different depths for different ceiling thicknesses.
I see a lot of work vans, cargo vans, with multiple 14 inch vents. Just gets too hot here for an electrician, plumber, A/C guy to work in his van without venting some of the hot air through the roof.