My ex informed me she will be taking our kids out of the state on vacation which will run over into my visitation time. I never gave her my permission and is leaving in three days, what can I do to stop her from taking our kids? I normally wouldn't mind however it will be the last week of summer vacation and I have plans with our children.

Probably the most expedient thing for you to do is to try and trade off some visitation time with the ex. Take the kids during the ex's time in exchange for her taking them on vacation during your time.

Did she discuss this with you at all in advance ? If not you have 2 choices. Drag her to court or get in writing a compromise where you get extra time some where else.

How much notice did she give?

Honestly, in reality, there probably isn't much you can do to stop her. You can, however, make her regret the decision. As soon as your visitation is missed, get back to court and have her cited for contempt.

Court orders are not negotiable without both parties being in agreement.