First, this is an awesome post that covers a great topic, and I’m inclined to move this out of the #help category because it’s typically for transient questions. Leaving that issue aside for now . . .
It’s not clear to me why you want your publish notes in a separate vault. And is there something about the publish feature that could be improved so that using one vault would be doable?
I’m not sure Obsidian will ever fully support nested vaults, but it’s definitely worth making a #feature-requests if this is important to you. I’m surprised an existing feature request doesn’t exist, but perhaps the devs response has scared that off.
If I used Publish, this is very much what I would do.
Anything I publish is carefully selected and organised, but is usually derived from a much wider network of writing and information. With Publish, I’d value the walls between the published vault and everything else, but I’d like access to everything else when I’m developing what will be published.
But easy for me to think that since I’m a heavy user of nested vaults.