It looks like :show-marks (or :marks, :list-marks) erroneously displays '(none)' for buffers that have one or more marks.
To reproduce:
$ echo "one\ntwo\nthree" | vile
and mark the first line (e.g.: with `ma', which displays `[Mark 'a'
set]' as expected).
Now try :marks.
I haven't bisected, though this issue seems to have been around for a
while (predating the current snapshots) as it's also present in my
system-wide vile (version 9.8 for darwin18.6.0, installed Thu Jun 27
06:54:50 2019).
