CPM is cost-per-thousand impressions while vCPM is viewable cost-per-thousand impressions.
A Display Ad can be below the fold and user might never scroll down to see the Ad. Or the user lands on the page with the Ad and immediately bounces off (in less than a sec). With CPM you will pay for these.
But with vCPM you only pay for viewable impressions. An ad is counted as "viewable" when 50 percent of your ad shows on screen for one second.