without investigating too deeply, i'm guessing that the module making part of slapt-mod can't handle the extended name (ie. vlc-3.0.20-x86_64-1ponce instead of vlc)
here's what i tried as a work around:
in /etc/slapt-getrc set the ponce repo as :PREFERRED (removing that tag from the salix repo)
then run slapt-mod -m vlc
this then produces a vlc module in /tmp as expected
but because the ponce repo doesn't provide dependency info it won't include any.. which would also be the case if the extended name worked.
anyway, thanks, this is something to investigate further, to hopefully make this script more robust.