Mm.

Van must be in relation to the rank of Legatus. Sol, as you've mentioned, is probably exclusive to the emperor.

There's also "nan". Like Midas nan Garlond, or Cid nan Garlond. I know Cid was named a minister after his father, and Midas was the Grand Minister of Industry if I'm not mistaken. So the "nan" prefix could possibly be in relation to the ministers.