Funny thing is if that's true, there is nothing to stop Liz from telling Vanessa to suck and keeping the $50k.
The obviously didn't sign a contract, and Vanessa would probably lose the stipend and prizes she did win if she tried to complain to producers. As Bunk noted, there are Federal laws regarding game shows, so Vanessa would have no legal action and could in fact get in trouble for trying to break rules.
That said, I really do think Vanessa was just bitter and emotional (surprise) when she voted for Liz. She really did play the best game and its gotta be a ball breaker.
That also said, BB is pretty selective in how they follow rules. Evel Dick openly admitted to cheating, that when he won his HoH, the letter from his brother was actually coded (they worked on a coding scheme before the show) with information and advice on who Dick should target.