There's a lot of different ways to skin this cat.
Making a flat spot via wood/foam, covered with fiberglass, would likely be best, but working with fiberglass and resin, on the center a Van roof, without lots of previous experience with fiberglass, could be a curse fest to end all previous curse fests.
While quality butyl tape is awesome stuff, it is not really intended to be stacked, to fill large gaps, and sikaflex is good stuff too, but again, huge gaps are problematic.
I'd probably masking tape off an 18x18 inch " square on roof, sand through the gel coat to green fiberglass below within this square. I'd peel the paper backing off some 1 " thick insulation foamboard, and use epoxy to adhere a 17.5x 17.5 inch square of foam to the roof, using weights to hold down the edges over the bend in the roof.
regular 'fiberglass resin', which is actually polyester resin, will dissolve most foams, whereas epoxy will not
Once the epoxy cures, come back with sandpaper on a long sanding block, and sand it flat. Then cover with fiberglass saturated with epoxy, then cut the 14x14" hole, then seal the interior of this cut out with more epoxy, thickened to almost bondo consistency with fine wood sanding dust, like that collected from a belt sander.
Once fiberglass sanded flat and smooth then butyl tape and install the vent and then use sikalfex or Dicor on the edges outside the butyl tape.
Epoxy resin and hardener must be mixed in a ratio precisely, by volume or weight, and then thoroughly mixed in the mixing cup. If one skips precision on this, they will regret it.
The foam can be replaced with wood, but it is harder to sand/shape and still needs the edges well sealed.
Cured Epoxy needs to be painted to protect from UV light. Polyester resin should be too, but is more resistant to UV.
Depending on how your roof is built and where you cut the hole, there could be some roof reinforcement required. so the flattening and fiberglassing can spread the load around the hole.