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
CPM bidding means that you pay based on the number of impressions (times your ads are shown) that you receive on the Google Display Network.
You’ll use viewable CPM bidding to make sure you only pay when your ads are able to be seen. Existing CPM bids will be converted to vCPM automatically, but it’s best to update your bids since viewable impressions are potentially more valuable.