Hi @annat - I would suggest starting here. We have lots of great docs available, and several detailed tutorials. i think spending some time working through those tutorials will get you up to speed on the ethos of Q2.
As far as your specific mention about vsearch in use in the the feature-classifier classify-consensus-vsearch method --- that particular method uses vsearch for classification purposes. As you learned in your other thread, this method leverages vsearch a part of the overall computation. It could just as easily have used some other tool or technique --- what I am trying to say is that even though this one method uses vsearch, it isn't the only place in QIIME 2 that you might see vsearch pop up! Some plugins in QIIME 2 basically just wrap a tool, while others perform very specific tasks, and others do a bit of both. By reviewing the resources I pointed you to above, I think that you can become familiar with QIIME 2 and the available plugins, and what it is they do.
If you get stuck or have any questions, you know where to find us! 