Is it legal to take away vacation time for any reason?

Going forward, an employer may change policies which means that they could stop allowing people to acrue more paid vacation days.  They may also put reasonable, business related limitations on people's use of paid vacations; for example, making sure not too many go out at once, no vacations during a critical period, shorter vacations (i.e. no one can go for 2 weeks at a time).  However, they cannot simply take away paid vacation days that have already been earned, since those were part of people's compensation.  Employees already performed work to earn them.

The only exception would be if an employment/union agreement allowed this; the employee handbook clearly stated that this was the policy (ie permitting losing accrued vacation without notice); or you were be singled out for a discriminatory reason.