Drag-drop as a complete file transfer into another vault.
Drag-drop in the form of an automatic link URL into a note of another vault. So the file you transferred stays in the original vault, but the link to that note is in the note of the other vault.
Easiest way of doing it would be putting both vaults into a folder; open that folder as a vault and then drag whichever files you want. When they’re part of a higher folder which is a vault, they act like folders. Read up on nested vaults.