The dynamics of this number is as follows:  

new VESTS can always be created by converting STEEM to VESTS at the current VESTING_FUND_STEEM to TOTAL_VESTING_SHARES ratio.

New STEEM is constantly being created; therefore we know that total possible VESTS must also grow
New STEEM is constantly being added to VESTING_FUND_STEEM at a rate 9x faster than it is being created outside the fund; therefore, the conversion rate from STEEM to VESTS is constantly falling.

We can expect the rate of VESTS creation to fall while the rate of STEEM creation is constant in absolute terms.

Eventually (about 1 year) STEEM will switch from being created at a fixed rate and start being created at a fixed APR. At this point in time the rate at which VESTS grows will level off.

The critical thing to remember is that growing of the number of VESTS never reduces the number of STEEM per VESTS.  

VESTS are diluted in percentage terms any time more than 90% of STEEM is held as vests.  

At the moment we have the following: 7652767.495/8477724.000:  0.902691276

This means that over 90% of all STEEM has been converted to vests. Most of the STEEM that hasn't been converted to VESTS is held in the REWARD FUND (786706.000) (9.27%) which leaves just 0.47% of STEEM liquid in people's accounts.

On July 4th, the REWARD fund will be distributed by the protocol, 50% VESTS and 50% SBD.  Assuming things don't change much between now and then, 95% of STEEM will be held in VESTS and 5% will be held in SBD (STEEM Backed Dollars).

The incentive to hold VESTS will fall dramatically over the next few weeks as the rate of change in the supply of STEEM falls.