Is there any way of importing a Scapple diagram into Scrivener so that when Scrivener compiles its output, I have a vector (hence high-quality) image in the output?
I have tried export to PDF and then insert linked file, but this still seems to embed a rasterised and pixelated image in place of the vector when compiling to PDF.
That is down to a limitation in embedding PDF in RTF files (from which compiled PDF files are derived, as it is Scrivener internal format)—basically, you can’t, embedded PDF isn’t supported by the specification, we are using custom code to hold the images as vector while they are in Scrivener. There are options in the Import/Export preferences tab for specifying the raster resolution, which should suffice for most purposes (the defaults are already high-quality 300 DPI, which is the industry recommended standard setting for print work).