So, here's what I think the plot is/was/has been:
The Police Chief and Caspere killed the jewelry store owners back in 92, and got rich stealing the diamonds. The daughter of the owners was Caspere's secretary who also went to his hooker parties. She is now missing. Seems likely she was the one who killed Caspere. The murder was caught on camera. When she went to get the camera, Colin Ferrell was there, and she had to buckshot him to steal the camera.
Caspere and the Russians and the mayor's son and the state attorney general and every other bigwig were doing something extremely shady with the train deal. Part of that was basically screwing Vince Vaughn out of land and his clubs. Early on, it looked like maybe someone killed Caspere to steal the 5mil of Vaughn's money. It turned out Caspere, et all, were already planning on screwing Vaughn. Him being screwed was incidental of the Caspere murder.
With Taylor Kitch dead, McAdams and Vaughn have to some how prove how crooked everyone is. That have no witnesses, a little evidence in terms of those contracts, but the one person at the state level who knew what they were working on got killed in her car.
Also, they could reveal who the little boy in 92 was. But mostly, it will be the 2 of them (or maybe 3 if VV joins them) trying to expose all the crooked big wigs.
It's not a terrible plot, but it is not worth all the work to figure it out.