Such a prosecutor, if there's anything at all to the claims of collusion between Trump and Russia, could likely flip Flynn into testifying about everything and against everybody in a bid for immunity.
(Flynn has already offered to testify in exchange for immunity.)
And if Flynn turned, it could begin a rung-by-rung climb up the ladder to Trump.
But even if Trump is actually guilty of something, such an investigation and prosecution isn't likely to happen.
We've got Jeff Sessions as our attorney general.
Sessions, of course, had to recuse himself from any investigation of Russian involvement in the election after he was caught hiding his own meetings with Russians.
But it remains unclear if he can get around that barrier when it comes to Flynn. And, at least according the above cited Vox report, it appears he can.
(At the worst, he could possibly do a Nixon vs. Archibald Cox and fire any prosecutor who went after Flynn as a potential witness for other wrongdoing. In fact he could pose as a tough guy and deny Flynn any immunity -- and Flynn's subsequent ability to pose a threat against Trump.)
Face it, Flynn is either going to escape any need for immunity one way or the other.
He'll probably end up paying a fine if he's convicted of anything.
And all the while it will be in his best interest to keep his mouth shut.
The only way around this is for a special prosecutor.
And right now, it's hard to see that happening.