Hi there, @sebastianvangreunen… I made an example that does what you described. The example starts with a custom state that holds the selected categories (note that I put the custom state directly on the index page).
Then, I have two workflows events associated with selecting a category via a checkbox in the category repeating group. One of the events adds the selected category to the custom state when the custom state does not contain the category. Here is the set up of that event and its associated action.
The other workflow event associated with the checkbox element removes the selected category from the custom state when the custom state contains the category, and I’m sure you can figure out the set up of that event.
Finally, when the Done button is clicked, a workflow action clears the list in the vendors repeating group, and a second action displays a new list via the following data source and advanced filter.
Note that there are no constraints on the search for vendors in the data source, and if you haven’t turned on the experimental parentheses feature, you should do that.
Hope this helps.
Best…
Mike