Add a vector arrow length legend
Include a legend which shows the meaning of the length of vector arrows. Probably by creating a single vector arrow with the value for that length, similar to the scale legend of a map.
There is a python script to create a legend available on pythonexchange.com.
Yes, it makes sense that this would only work orthrographic mode or certain other limited cases. This feature could be seen as part of a feature set and usability considerations which is aimed at users who need to present clear 2D images for reports or academic papers. In such cases the figures need to be self-explanitory with the meaning of all parts of the figure clearly identified.
That's an ok idea for a 2D problem viewed in orthographic mode. But in 3D it's mostly useless because (a) the vectors are in 3d and could be pointing in/out of the screen, and (b) perspective is on by default meaning things that are further away from you are smaller so no way to compare to a legend - in fact having a vector arrow legend displayed while in perspective mode could be very confusing.