Since Vivid allows you to create up to 15 sub-accounts with their own IBAN, there is of course no single IBAN. To find out the IBAN of a sub-account, simply select the desired pocket in the pocket overview of the Vivid app.
Then click on the cogwheel symbol at the top right. You can then customise the name and preview image of the pocket as well as view the IBAN and BIC information.