Sara Paretsky's character, V.I. Warshawski, is a tough-talking; tender-hearted; justice-seeking; single-minded woman who is approaching middle-age and beginning to feel it. I can really identify with the "beginning to feel it" part, which is part of the reason I find her so engaging. Her tactics may not always be perfect, but her heart's always in the right place!
In Tunnel Vision, once again, she goes after the truth taking her friends, her dogs, her police contacts, and her neighbor/protector, Mr. Contreras along with her. She discovers the truth, too, but only after putting herself and her closest personal relationships at great risk.
I don't think she's meant to be believable 100% of the time, after all this is fiction. V.I.'s just supposed to be cool. And she is.