Sky is oddly over here grinning at that.

Anyway haha like I said most of the police officers are slaves for vampires from one or another of the houses. So I'm sure she can get some of the vampires to believe her, if nothing else she has the marks. Where Grace is from wouldn't be Ravensblood ruled it would be right outside (I realized from rereading my intro) it would be Darkcrest. Which would be why Skylar would rush over to get her. 

Though Skylar wasn't the only one out that night, and she's the only one that doesn't have a slave so. Of all the people known to be attacked (there were some that were turned) Grace will be the only one still alive. If that helps you have a reason to believe her.