Speaking as someone who was sexually assaulted, yes, tracking the person responsible down is unethical. In my case, they filed it under miscellany and never interviewed him, and less than a month later, three more cases turned up and the guy skipped state. The police never followed up, and so he's very likely still out there. I did the responsible thing and contacted the authorities once I got out of the hospital. Granted, I did it while staring at the floor, stuttering, and being held by a friend, but I did do it.
Can I say I'd do it the same way if it happened again? No, not really. It was a traumatic experience and I won't sit here and say if given half a chance I wouldn't have returned the favor at the time. But I don't think two wrongs make a right, and feeding this guy to a woodchipper because the police wouldn't do their job, while maybe emotionally fulfilling, isn't right.
If there is a such thing as right and wrong in a scientific context it must be based on the result of the act and not the emotion or nature of it. If reporting the rape to the police results in the rapist going to jail then that was right. If the police can't catch the rapist then you have a responsibility as a victim to do everything in your power to track the guy if you have the resources to do it. As long as you don't break any laws or hurt any innocent people in the process, it's ethical for the victims of crime to track down the individuals who victimized them. If the guy commits another rape then you got him, if he doesn't then you can blacklist him but the point is he must be stopped one way or the other if it is to be an ethical result.
So I have to stand by what I said -- vigilantism is not a public service. A public service would be my day in court, along with the others who that son of a bitch hurt. So I do understand the motivations behind such behavior on a very personal level. I don't agree with it.
If the rapist does not respect your civil or human rights, why respect his privacy? It all depends on what the law says, what you can get away with, the resources you have, the whether the police are corrupt or not. You cannot always depend on the police is what I'm saying. And no being a vigilante is not the answer but neither is depending entirely on the police. At some point you have to protect yourself.