How to Unlock You’re under arrest! Achievement
As much as I know, Vincent is the dude who sits in the air vent on the end of the town level. The problem is, that talking with him as a cop, doesn´t make much. If you talk with him as another town member, you can accuse him of being at a crime scene earlier but that´s really all.
The only progress I have reached so far is that when I talked with him as an elderly man (who you can also see in the picture below) and then as a police officer, his air-vent-thing moved a little bit. But once again, that might be just a bug.