Legal Question in Employment Law in California

It is illegal to take away earned and accrued vacation time. If, however, the vacation time was erroneously credited in violation of the vacation accrual terms of your employment, such as if there was a cap on how much you could accumulate without using it, then taking that vacation time back is legal. In short, the rules are rather nuanced and the legality of what was done will depend on the exact terms of your employer's vacation accrual policies, and exactly what happened. So you need to talk to an employment attorney in person and go over the details if you want a definitive answer.