Currently there is a big button to create a new vault. If you press [meta]-q, you see the Activities selector which also has "new" button, but the button is just a plus and text. It doesn't really look like a button. The approach seems to be a button that on mouse-over has a button-look, but otherwise it doesn't. Similar in the "add widget" plasma widget you get from right click on your desktop (after unlocking widgets). Please consider following that approach in order to be consistent.
The UIs really aren't comparable IMHO. Vault's UI should follow what the other small square system tray popups do, since it's one of them.
This seems like a WONTFIX; the new New Vault UI that went in with https://cgit.kde.org/plasma-vault.git/commit/?id=fef23f48913f1c7027f50da83e6137bb27bab10d has this being a normal button, to match other plasmoids (like BLuetooth). We now *are* consistent--consistent with other Plasmoids.