In the web app it seems that it's only possible to move one item at a time between vaults. When moving several items in a row the web app sometimes crashes and I have to reload the page and login again. The last time it crashed was after moving somewhere around eight items. When it crashes, the message in the image below remains on screen and the spinning symbol just keeps spinning until I reach my time limit for the session and get logged out.
When I've logged in again I can see that the item that I was moving when the app crashed is duplicated and each of the vaults I was trying to move the item between has a copy of the item.
1Password Version: 6.1.272d
Extension Version: 4.6.1
OS Version: Windows 10
Sync Type: Families