Is it possible to have LayOut default to vector mode rather than raster?
No. There isn’t a setting for that. I once thought it would be nice to have that option but Vector (and Hybrid) rendering takes longer to process and with some models really create a huge delay in getting the LO project started when the SU file is inserted. If you set the render to Vector for the first viewport and then copy it for additional viewports, all of the subsequent ones will be rendered as Vector, too.