Think of Vests like Shares in the Steem ecosystem. They differ from Steem because they don't get diluted with extra Steem being printed, however they are made up of Steem themselves. Probably not a great explanation, try this...

https://steemit.com/interest/@sigmajin/understanding-the-steem-economic-system-vests-sbd-steem-dilution-interest-and-all-those-crazy-things