Vanessa Tyson, the woman accusing Virginia Lt. Gov. Justin Fairfax of sexual assault, is a political — and perhaps legal — wrecking ball for the Democratic rising star who until this week was positioned to take over as governor. Fairfax risks not only his political career but also criminal charges, with Tyson alleging an assault at the 2004 Democratic National Convention in Boston, where a 15-year statute of limitations allows five more months for Tyson to pursue the case.