Hi Christoph and welcome to the Community!
Qtrees are basically folders/subdirectories within a NetApp volume - you can share a qtree, instead of a whole volume. You can also apply quotas to qtrees
You can also put a LUN inside a qtree, but it doesn't make much practical difference - unless you use SnapVault and/or Qtree SnapMirror replication.
VServer is specific to Cluster-Mode only (there is similar thing called vFiler in 7-Mide) & it is a sort of a virtual filer - you can do logical separation of your resources using it.