I'm having some problems with vanilla interiors not properly displaying in the renderer window. NiptonTownHallInterior, and NiptonTownHallInterior3. NiptonTownHallInterior2 is fine.

Is this an issue with the portals and room markers, which make it so that only that what can be seen by the player is rendered (or was that something else?)? How do I fix it? No matter how I look at it, nothing appears except just a couple of statics.

I am a bit astonished we discussed it here about a dozen of times now (sticky????). Yes the problem are the portal & roommarkers which are affecting strangely GECK.
Simply move them out of the way ( z+5000 for example) press F5 to refresh the render window, make your changes and then put them back in place.
The last step is to clean the changed room & portalmarkers with NVEdit.

That's pretty awkward, I wonder if Obsidian had to deal with this too? But I guess they added the portals last and then didn't have to do anything with the level anymore...

It seems pretty pointless to me. It's not like any level in New Vegas would be too heavy for the GECK to display at once, especially since it doesn't have to calculate NPC AI and sounds and all that.

Sure it's pointless and I assume it is a bug that has occurred while they have taken out something for our 'end - user' version. It's similar with the havok and light problem.

Something about the 'module' which handles the render window isn't correct.