Hi oneself, as you probably know, deb-multimedia is not an official debian repo but driven by a single person, Christian Marillat. Some people think they need to use it because of special versions of packages or programs they cannot find elsewhere.
But as this repo is not maintained by siduction nor by debian, if something breaks or dependency mismatch occurs it is up to Marillat to solve this (or up to you to not mix his repo into your sources.lists).
As for siduction, all I can say is that vlc and vlc-nox are working here with stock siduction sources, meaning debian sid + siduction repositories:
# LANG=C apt-cache policy vlc
vlc:
Installed: 2.1.2-2+b3
Candidate: 2.1.2-2+b3
Version table:
*** 2.1.2-2+b3 0
500 http://http.debian.net/debian/ unstable/main amd64 Packages
100 /var/lib/dpkg/status
#LANG=C apt-cache policy vlc-nox
vlc-nox:
Installed: 2.1.2-2+b3
Candidate: 2.1.2-2+b3
Version table:
*** 2.1.2-2+b3 0
500 http://http.debian.net/debian/ unstable/main amd64 Packages
100 /var/lib/dpkg/status
As a first step, you could try if " apt-get install -f " has a solution.