This repository contains a few routines that I follow when preparing the images with VMD
Copy and paste the content of the "vmd_draw_vector.tcl" in the VMD TK console and then you can draw arrows with the following command:
draw vector2 {a b c} {d e f} x y
where a,b and c are the coordinates of the starting point of the arrow; d,e and f are the coordinates of the point that defines the arrow direction with the previous point. x is the length of the arrow and y is the resolution of the arrow. Full credit to the original tutorial, available at http://www.theochem.ruhr-uni-bochum.de/~legacy.akohlmey/cpmd-vmd/part7.html
After a VMD rendering it is probable to have a very large image. To reduce its size from several MB to a few MB just open it in Inkscape and export it as png without changing the settings.
display resize 500 600 (for example)
then use the following keywords (plus the default ones) in Tachyon
-aasamples 12 %s -format PNG -o %s.png -res 2000 2400