"vec off", "vec on", or "vec arrow".  With "arrow",
draws an arrowhead on each vector, with 3-D size equal to
the vector's length times the 'arrowscale'.  The arrowhead always
lies in the screen plane, so its size gives a cue to the vector's
true 3-D length (e.g. when the vector is viewed nearly end-on,
even a small arrowhead can look longer than the vector does).
This also means that arrowheads flip orientation when a vector
passes through being seen nearly end-on.

Arrowhead size is set by a new second parameter to "vecscale".
Default is 0.25, meaning that arrowheads are a quarter the main vector's length.