Vests are shares in the Steem Power Fund. If a user takes no action on the Steemit Platform, his/her shares will remain static in quantity, just like shares in a financial fund. Also, like shares in a financial fund, the underlying asset base can change with new investor’s joining, other investors leaving, or fund performance being good, but non of these action change the amount of shares you hold in that fund… (source : @hisnameisolllie)