Viewable impressions allow you to:
A) pay for those ads that are actually viewed
B) pay for ads that are viewed and also converted
C) pay for video ads that are viewed within an hour
D) pay for ads when they are displayed in a viewable position
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