Is it possible to split inventory items? Meaning, let's say there's 40 of something in my vault but I only want to grab 10 of them. I don't see a menu item for splitting them?
Press shift then click on and drag the item to another inventory slot. When you release the mouse click it'll give you the option for how many to split.
And if that doesn't work, left-click and drag the stack, then hit [Shift], then release into the new spot.
(Note that this is only guaranteed to work within your "bags", not from vault to bags or vice versa. So if you want to take only "some" of a stack from your vault, you need 2 open pack spaces - one for the whole stack, and another to split into - and then return the unwanted portion to the vault.)