As I indicated already, you might have had some incompatible mix of libraries installed.
In particular, as you installed vlc-codecs using the VLC repo first, you might have had packages from there (ffmpeg’s libav* e.g.) which are incompatible with the packages from Packman (different versions).
Is packman repo aimed at a particular Linux distro like openSUSE?
Mostly, but not exclusively.
What makes it different from official openSUSE repos and why it is not added when installing the os?
Well, it is maintained by different people.
It is also not backed up by a company and located in a different country where other laws apply. So it can host things (multimedia-related in particular) which official openSUSE servers cannot.
In the end it’s all a legal thing, mostly about licenses and patents.
It’s not added automatically, because that would be a possible legal problem as well.
But you can find it in the so-called “Community Repositories” list, YaST->Software Repositories->Add->Community Repositories.
Is packman "on your own risk " repo and does it contain stable packages?
Well, everything is “on your own risk” actually, read the openSUSE license…
But yes, they do try to provide stable packages. And as it is the biggest third party repo (and exists since 10 years or so), it is safe to use.
If there are more repos containing the same packages, what logic does zypper follow to chose the package?
Normally it just takes the one with the highest version number. You can also set priorities to repos, then it takes the packages from the one with the highest priority. You should only do this if you know what you are doing though.
And for updates (zypper up), it does not switch packages to versions from other repos.
You have to use the “zypper dup” for that, which actually means “Distribution UPgrade”, but with the --from switch can be used to force a switch of all packages to a particular repo. The same you can do in YaST btw.:
https://en.opensuse.org/SDB:Vendor_change_update#Full_repository_Vendor_change