In the process of divorce and we are still cohabitating until the divorce is final. We are going on vacation and I want definite dates in writing for the amount of time I will have the kids. If she does not want to sign a form and get it notorized can I prevent her from taking the kids on vacation?

Unless you have a standing child custody agreement or a child visit agreement, you both have rights and either parent can take the child where they want, and when they want.

And a "signed letter" that is "notarized" has not legal standing, unless it was taken to the court and approved by the judge. She can write it up, sign it, notarize it and then just ignore it and there is nothing you can do about it. You can complain about it, show it to the judge when you get to court for custody latter on, but it will not have much value.