You must first have a Premium Vimeo account, as Vimeo does not allow posting from other applications unless you have a paid subscription.
Next, you'll have to link your Vimeo account to your Pixorial account. You can do that by logging in, going to your email address at the top of the screen and then Social Networks. Follow the prompts to connect your Vimeo account.
Once the accounts are linked you can select any video in your library then select "Action" and then "Vimeo." A pop up will appear asking you to input a title and description, then you can send it off. It's that simple!