Hi there !
I would like to share this tought and know your opinions.
I was thinking about how my ZK (or second brain or permanent notes) could be completely separated from fleeting and literature notes.
I think they should live in different vaults, or even that fleeting and literature notes could even be in other systems.
Fleeting notes could be also spread across several sources (paper, bookmarks, google keep).
In fact, they should not even be linked to each other, since they will be linked when I consolidate them in my zettelkasten (which of course will be in Obsidian).
I think having everything in the same vault would make a mess of notes when I search for things, having temporary (which at incubation state could be even wrong information) mixed with permanent notes.
Also fleeting and literature notes may follow a different organization than my ZKā¦ maybe literature notes could have more āstructureā to follow the source structure, for example.
At this point I think Obsidian is really good to manage my āsecond brainā, but I start to see fleeting and literature notes only as āsupportā stuff, which could even be more varied than those two.
Do you think that could be a good approach ?
I do have something similar with separate vaults. But those vaults are also part of a system of highly nested vaults, so I can easily switch between linked and unlinked states.
What matters most is that your system makes sense to you, because then youāll understand when and how to change or tweak it. Starting with someone elses system leaves you permanently wondering if problems are simply down to your lack of understanding. And I see no reason why your suggestions couldnāt work for you.
As Dor said, whatever makes sense to you! For me personally, I would want to be able to link in some easy way to my literature notes as a form of citation, but also my particular note-taking goals can result in notes saying something like āCitation1 solved this variant of the problem and Citation2 solved this other variant, and Citation3 claimed to solve the whole problem but did not include the variant from Citation2ā. These are really useful to me later, whereas they might not be useful at all to others!
Ultimately, you do you! That saidā¦ 
A zettelkasten isnāt really a second brain. A second brain contains all sorts of stuff thatās really not included in the ZK. To-dos, people, birthdays, you name it. Itās literally anything.
A zettelkasten is a container comprised of multiple compartments (zettels, lit/ref notes, etc). Each has its own folder/drawer. And, because the captures on the lit note will often link to the zettel made off of them, thatās why having them in the same vault is key.
So, one way to think of the SB + ZK connection is that yr ZK is an aspect of yr second brain.
Again, whatever works for ya. But, thought Iād share that in case it helps.
Thanks @bobdoto
So ZK is for āintellectual knowledgeā and a SB would be ZK + everything else ?
And wouldnāt be better from a zettel to reference the original source rather than referencing my own lit notes ?
I mean, the lit note is āmy interpretationā of the topic, which could be wrong depending on some context.
I donāt know, itās hard to get clarity on all of this 
You are not alone in the confusion! Thereās a lot of contradictory stuff out there. If I may helpā¦
Your lit note isnāt your interpretation of the material. Itās simply a reference to help you find what caught your attention in the source (book, pod, vid, etc). Hereās a short piece I wrote on lit notes that may help.
You can absolutely link back to the source, but for some itās just easier to quickly [[]] to the lit note, which will contain the bibliographic info, and leave it at that.
As far as zk/sbā¦
Your zettelkasten is for creating a linked network of your ideas to help you better develop them for eventual output (writing, etc). You donāt have to be a writer to use a zettelkasten. But, you at least want to have something youāre working on, be it specific ideas, etc. Itās a development tool.
Your second brain can contain that (yr ZK) and literally anything else you like. Same as your real brain. 