My guess is that few people who dwell in vans take the time to see how it is done in RVs, boats, airplanes, etc. Otherwise there would be much more common knowledge of the easy solution to hanging curtains from the ceiling. So take notes, bookmark, remember that there is a low profile, alternative solution to using a big bulky metal rod.
In those other vehicle situations they use conformable to a curve, ceiling mounted curtain tracks. Small slides fit into the track and the curtains are hung from that. You need the curtain track with a flange on one side so you have a place to attach it with screws into the structure of the van. It is very low profile in height which is a big advantage as you won't be hurting your noggin on it. The track is generally made from plastic but with some searching it can be found made of aluminum. You will also need to order the stops which keep the slides retained in the track at the two ends. Here is link to an image of the profile of the track plus enlarged views of the slide and the clips which hook into the slides.
Cutrtain track plus fittings to go into the track.
Keep the curtain material fairly light in weight. It can be an insulating material such as acrylic fleece. Fleece is not heavy but it is nice and thick and blocks both light and drafts. Insulation is useful in summer and in winter to control which side the heat is retained in.
Keywords" RV ceiling slide track".
Be sure you also order the slide attach pieces. There are several styles of it. For instance one type has the end that slides into the track and then small spring loaded clamps hook into those slides. That allows for a no sewing needed curtain installation that is easy to remove and launder. Here is a link to a profile of the track and enlargements of the slide and hanging clips.
You can get The RV ceiling slide track at most RV supply stores which is the best option for saving on shipping cost as it comes in 8' pieces. If they don't have it in stock they can usually get it delivered to their store from their warehouse distribution center in two or three work week days.
If your van has a plywood or other type of firm ceiling board then you can pull rivet the ceiling track to the ceiling board.
Amazon ASIN product number for the track is B002UC2TFK