Unfortunately, this type of abuse is essentially only acted on if you either (a) cause some kind of problem for ops at the VPS company, or (b) the victim tells on you. Even then, the absolute worst thing that will happen to you is your account will be closed. There is just no way on the modern internet to investigate/prosecute these kinds of things. That's why one of our primary goals with the free Cloudflare plan is to invert the problem: make DDoS go away by making it it free to mitigate.