it’s not recommended mostly for the fact that it might not be compatible in future versions. So far there are no issues (that I’m aware of)
A Vault within a vault is only a way to open a subfolder inside another vault, and open that subfolder as it’s own vault (from the opening vaults menu)
I also wrote another post on this if you want to see more disccusion on it.
I would say, if you don’t see an obvious need for using nested vaults in your system, then perhaps you don’t need them. it’s not a bad idea to avoid them, since we don’t know if future version of obsidian will support them