since the VMs includes bits and pieces from several
scientific disciplines, it might well be that the
diagram is not astronomical or astrological at all,
but rather meteorological. It would make perfect
sense to have an eight-fold division for the
eight major directions from which the winds might
come. The figure is of course rather unusual in
that sense, but so is the whole MS :-)
I don't know anything about treguenda, or how
old that tradition is, but there is one aspect
I like about that explanation: the VMs figure shows
four larger sprials and four smaller ones,
alternating. Also the writing direction
alternates.