Introduction8-Ball has discovered that Vinnie was due to meet a bartender called Jonnie at the docks. He thinks it might be a lead to find out who killed him.
Pick Up The BartenderHead to Portland Harbor in a vehicle and meet the bartender near the water.
The bartender will tell you of his surprise at Vinnie's death, but tell you that Vinnie owed him money, and as his partner, you can make it up to him.
He wants to be dropped off at his bar, so make your way there. As soon as you start to leave the docks, a bunch of gangsters will show up and chase you. Run over a few of them and you'll spot a few cars. You either need to lose them or blow them up. I was lucky in that I crashed into one of their cars which flipped over and blew up the other two.
Once you've gotten rid of them, you'll be asked to take him to the bar.
As you arrive in the Red Light District, he'll tell you some people want to kill him, so you'll need to take them out. There's a few to the South, and there may be one or two who will attack the car.
This part of the mission is quite a pain, and I actually failed it multiple times for various reasons. A few times I was arrested, and others I accidentally shot the bartender...
The best solution seems to be to get straight back into the car, and use that to run over the enemies.
Once they're all dead, Jonnie will tell you he'll speak to you soon.