Once, draw particle velocity was set, it allocated an array for the vector data. After that, it was only checked if the vector data was allocated to draw the velocity by lines. Simply checking for part->draw & PART_DRAW_VEL in addition does the trick. However I am not sure, if that draw data should be freed too, if not needed or if there are any cons (I haven't read enough code there to make a judgment).

Aurel