i want some custom fields in the discussions, which depend on the chosen category,
would this be in any way possible?
Sure, but the best way to do this depends on what you really want to have.
a) Do you want always the same number of fields per category or would that also vary?
b) Will this be a required field?
c) Do you want to sort by that field or search for it?
b) Are this only one or two fields or will there be numerous different fields?
I ask because you can extend Vanilla in different ways. There is a column in the Discussion table, that holds "meta" information which you could use to store some values. But it is not suited if you want to search for that information.
You can add columns to the discussion table. If you always need the same number of additional fields, it might be best to use that approach and only change the label for a field when you create a new discussion.
If you have different and numerous additional fields per discussion, a separate additional table would be best.
I guess that creating such a plugin could be done by a beginner, so if you like to do so, just start and ask whatever you like.