In multi-vault mode, there is an option to select the vault where new items should be saved to (Settings > Vaults > Always Save Items to Vault). I do understand, that this is necessary when using "All vaults" mode (icon left to search field).
When selecting a specific vault though, I would rather like to have the selected vault also be pre-selected in the "Add Item" dialog. Having selected a different vault there already got me a few times, when adding a new entry at work and later missing it at home just to find out that I saved the entry to my work vault instead of my private one.
My suggestion: Add an additional option (checkbox) with which we can switch between "really always use this as default" and "only use the default in 'All vaults' mode, otherwise pre-select the currently selected one"