You cannot remove vlc.
- plasma-workspace depends on phonon-qt6
- phonon-qt6 depends on phonon-qt6-backend
- phonon-qt6-vlc provides phonon-qt6-backend
Being creative you could create a dummy package providing phonon-qt6-backend - but the consequences should be researched and carefully considered.
Due to the gap between Manjaro stable branch and Arch proper - a gap that lastet 2 months when Plasma 6 was released - it is recommended to not use AUR.
However using testing or unstable branch you would have the same issues or lack thereof as any Arch user.
It is important to remember that certain dependencies will have different naming - and you have to be able to choose the correct packages - e.g. linux-headers which is a dependency of dkms - can only be resolved correct on Manjaro by pointing to the relevant linuxXY-headers package.
As for vorta - it has been moved from AUR to the repo and is currently in unstable branch