firstly, thanks for the great support.
I wondered if there is a smart way to separate the users that register into two groups (f.e. through separate forms or a checkbox) so they have a different role/right-concept.
It should work this way by default, vendor is just a user with at least 1 listing (you can also allow direct vendor registration without listing anything). Users with vendor profiles get the Contributor role by default so you can distinguish users this way (e.g. when sending out a newsletter).