As I use Obsidian for my full PKM system, not just the Zettlelkasten part, I keep everything in a single vault (everything = all evergreen notes, I call them notions, regular notes, bookmarks, and project related notes.), but I have separate folders for them. The evergreen notes have a separate folder, away from the dynamics of the work and information input that swirls around it in the other folders. I also have a folder in the evergreen folder for ‘proto-notions’, which is stuff that will become an evergreen (or get ‘demoted’) over time, but needs work still.
I keep all of it in one vault explicitly to be able to link between things. So I can link to relevant concepts when I am in my ongoing work notes. So I can link specific examples I found to the evergreens they are an example of, or vice versa. For regular notes I usually add a few words about why I think something is of interest. That why often relates to an evergreen concept, so I link to it as well.