Sounds like your garden variety trainwreck/basketcase. The significant majority of those types do nothing more than create drama and engage in self-destructive behavior. Without specific threat information I wouldn't be overly concerned. Never stop being vigilant and have a plan in case something happens at work, but I wouldn't burn vacation over the issue at this point.
With that said, always trust those little hairs on the back of your neck!
+1. I wouldn't go freaking yourself out too much about it. If she's that crazy, she probably wouldn't even be able to put two and two together that you would've been the one to disprove her lies trying to cover up her terrible productivity. I wouldn't certainly hope your boss/head of the company didn't just call her in and say something to the effect of "We checked, and, uhh, sixseven says you're full of B.S."
Maybe you're just more concerned that she's gonna freak out and go postal, so to speak?