Hi kbd, I found your topic when searching for the same answer and I’ve found how to do it.
This works for all types, logins, cards, identities, secure notes.
For the login you’ve already made, go to Logins and tap on the … to the right of the login. Select View, then the horizontal … in the top right corner, then Collections, then you can choose which Collection you want it in. This won’t take it out of Logins if you’ve got a Family account.
To make a login so it automatically ends up in Collections, when you’re in My Vault, select the Collection you want then click the + sign. At the bottom of the Add Item page, you can select which Collection you want the item in.