Allow you to only pay for those ads that are actually viewed
Allow you to pay for ads when they are displayed in a viewable position
Allow you to only pay for video ads that are viewed within an hour
Allow you to pay for ads that are viewed and also converted
With vCPM bidding, you bid for your ad based on how often it appears in a viewable position on the Google Display Network.
Read more here: https://support.google.com/adwords/answer/2630842?hl=en