Yeah this is exactly why I started with them in a single vault and also exactly why I ultimately split them. Your point about the vaults having gravity is absolutely spot on, that’s exactly how it felt when I made the decision.
As soon as I did that they began rapidly changing and evolving into their own unique structures suited for their independent purposes. My projects vault now has a completely different structure and workflow than my ZK.
For example, the projects vault is primarily organized around the day while the ZK is organized around topic outlines (e.g. MOCs). There’s no daily notes at all in the ZK but in the projects vault there is one for every day. Meeting notes and “significant action” notes (important emails, decisions, etc) are tracked in notes right alongside the daily notes in a “Timeline” folder. All notes in the timeline folder start with the date in a particular format e.g. 21.05.May.03.Mon – meeting/etc notes then become 21.05.May.03.Mon (M) Discussion about blah blah. Within the note I include lots of links to people-specific notes, which in turn use dataview queries to generate lists of notes where they are included in specific fields or just mentioned.
None of that is in my ZK, which is very manual and focused on deeper knowledge and insight generation. Project vault holds things specific to things I do at work, not expected to be stored permanently. When they are no longer needed there the ones that are worth keeping will probably shift into my ZK. The ZK also holds concept & theory notes which are then used to help structure strategies & projects in the work vault.
There’s essentially zero links between them. A few times I mention a name in the ZK that is in the projects vault, but don’t link to it, and a few times mentioned in the project vault something like “see topic X in ZK” but that’s it.