A prosecutor can still bring a defendant to trial even if a grand jury does NOT vote to indict.

It's probably unlikely to happen in the Ferguson case, but it is possible.

So if they don't vote to convict, we must press for the release of all the evidence presented to the jury. And if there are significant errors or omissions -- which appears likely -- we should demand a trial regardless of the grand jury outcome.

We also should press for a Federal trial, as with Rodney King.

http://criminal.findlaw.com/criminal-procedure/how-does-a-grand-jury-work.html

The Grand Jury's Decision and a Prosecutor's Discretion

Grand juries do not need a unanimous decision from all members to indict, but it does need a supermajority of 2/3 or 3/4 agreement for an indictment (depending on the jurisdiction). Even though a grand jury may not choose to indict, a prosecutor may still bring the defendant to trial if she thinks she has a strong enough case. However, the grand jury proceedings are often a valuable test run for prosecutors in making the decision to bring the case.

If the grand jury chooses to indict, the trial will most likely begin faster. Without a grand jury indictment, the prosecutor has to demonstrate to the trial judge that she has enough evidence to continue with the case. However, with a grand jury indictment, the prosecutor can skip that step and proceed directly to trial.

- See more at: http://criminal.findlaw.com/criminal-procedure/how-does-a-grand-jury-work.html#sthash.X1EyhQw3.dpuf