An Vividi ID is a unique identifier located on your profile, under your nickname, and that can only be seen by you. It can be used to add your friends by going to the 'Message' tab > 'All' Friends > tapping on the '+' icon at the top of the screen > select 'Add friends by Vividi ID'.
You can make an Vividi ID by clicking on the Vividi ID icon located next to your nickname on your profile page. Please be advised that you cannot change it once it's been set.