Well, while I do not know who the vigilante, doc or cop are, I would like to offer them some advice for how i would play tonight.
Cop: Well, it is quite simple, assuming you survive today (which is likely) you should investigate a prominent figure (Thucy, Jok3R, Chaqa, Krellin, Warden) you are unsure about. Such prominent figures tend to steer conversation, and while it may just be a very expressive townie, I have a hunch at least one member of the mafia is part of the main conversation. You could also look for a not so prominent figure who posts near the end, as we have previously expressed this as another mafia strategy. Also, I suggest you try to influence conversation after you have some hard evidence, but don't reveal your identity willy nilly, because if the mafia knows who you are, you will be either role-blocked or killed.
Doctor: It is likely the mafia will go after a prominent figure, so I suggest you find someone you believe is in danger and is near center of conflict (and who the mafia might want to lynch, obviously) and protect them. However, I do not think it would be wise to protect someone directly in the center of conflict, as I have expressed my previous concern that at least one may be mafia(or even the serial killer). Granted this is mostly speculation, so take my words with a grain of salt.
Vigilante: As I suggested before, I suggest you focus on finding the serial killer, as he can shield himself from investigation or death, so he will be tougher to catch. Also, as Fasces mentioned, don't waste that shot.
@Fasces: While you may not use math to play mafia, probability and risk management can not be ignore. And while I did say algorithm, I meant process based upon logic, reason, and rules, but algorithm sounds so much cooler. If you believe me to be suspicious, I will happily sit for a trial, but that will waste time. Despite your accusation of me, I believe you actually are trying to benefit the town, so I will not cast my vote for you. I may wait a little longer for more posts, but I have a good idea on who I think actually is mafia.