Love Boat

This could be a lead to find out who killed him.

Pick Up The Bartender

Head to Portland Harbor in a vehicle and meet Jonnie the bartender near the water.

Jonnie will tell you of his surprise at Vinnie's death, but also tells you that Vinnie owed him money, and as his partner, you're responsible for making it up to him.

He wants to be dropped off at his bar in the Red Light District, 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 who give chase. 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, but if you drive down some tight alleyways it's pretty easy to lose them.

Once you're free of the goons, you'll be asked to take Jonnie to his bar.

Red Lights Spell Danger

As you arrive in the Red Light District, Jonnie will 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 more who will attack the car.

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.

Mission Passed!