“My sister has an ex-boyfriend that she believes has slashed her tires. She got a trespassing order against him. Then 2 days later her car was vandalized. What can we do? No one is coming forward as seeing anything. She is pressing charges but is it going to be pointless because she has no proof? Is there a way to open an investigation or something to actually get proof that it was him and he was not elsewhere when the car was vandalized?”
Under the legal concept of “discovery”, once a case is open, investigation can be done. However with a criminal case your sister is not actually the plaintiff, the state is, and to a large extent it will be the district attorney or other state prosecutor who will be driving the case. In the scheme of things and the nature of crimes with which a state deals on a regular basis, it’s likely that the state is not going to throw a lot of resources at a vandalism case. They simply have much larger problems (armed robbery, murder) to deal with, and limited resources.
If your sister comes up with some supporting evidence, the prosecutor should be interested in it (provided that she obtained it legally!), but a much surer, and less expensive, way to handle this is to let the state do what it does, and to tell your sister to park her car somewhere secure from now on.