Ivarson - Thank you for your comment. Yes, as you say, there are various advantages to both approaches. The reason I thought one-way, could be more viable, is it wouldn't necessarily require any syncing, as entries are purely shared with multiple vaults. The other advantage of one-way is in a family set-up, it would ensure there can only be one version of a particular entry, while still allowing secondary vaults to create and control their own unique entries.
The absolute ideal would be to also have a master list, collating every entry from every vault, giving the account owner an overview of the entire system.
Anyway, thanks for your thoughts and here's hoping.