U-bolts. I drilled holes through the solar panel frame, installed the flat bar that comes with the u-bolt on top. Seven years ago. The holders for sliding out the lower panel were U-shaped scrap aluminum patio door sections.
For the roof vents I just took the cover off and the solar panels will prevent water from coming in. If you already have roof racks, I would just use those, unless they are flat on the roof they should easily go over any roof vent (minus the cover).
On my astrovan I prefer using one large panel versus a bunch of smaller panels. I got a 365 watt panel that has served me well for the past 3 years (maxes out at 27 amps), enough to keep my 500 ah of lithium fully charged to 100 percent every day. I had a 120 watt panel installed when I first went solar, it was installed sideways and it was sticking out over the edges which was an eyesore to me. The 365 watt panel fits nicely over the roof, none protruding over the edges. I didnt have any roof racks so I had to install 4 L-brackets to mount the panel. View attachment 34106