I just added a vault and now Dendron is creating notes in the new vault

When you add a vault, you change your workspace to a Multi Vault Workspace. The default behavior is to create a note in the same vault of the current opened note. You can switch to a note in other vaults by opening a note there (eg. if you type root, you'll see a list of all vaults). You can set confirmVaultOnCreate to have Dendron always prompt for a vault when creating a note.