This is a hard situation to be in. I know because I have been there.
I had a friend who I had to cut ties with completely. She was very demanding of my time and her emotions were a bit on the volatile side. Long story short, she got the impression that I wasn't going to hang out with her anymore and totally freaked and threatened me. That was it for me, I cut all ties with her, didn't answer her calls, blocked her from my email etc. She kept trying to contact me. Even after over a year with no contact whatsoever, I still get the odd invite from facebook from her.
Anyway, I know it is sometimes hard when you want to keep a friendship but they make it hard to do so. You want to be able to forgive and forget, but it is not always that easy. I don't think you should lie to her and as hard as it will be because I'm sure you will worry that she will show up and make your vacation difficult, don't let her. Go ahead with your plans. As some people here have said, Disney is a huge place, it will be hard, although not impossible, for her to find you. Don't let her win if it is a game or power struggle she is at.
Also, I think it would be wrong and selfish for your husband to blame you for her actions when you have told her your wishes. I think he needs to see that or it will be him who is ruining the vacation. You can't control her actions.
If you do want to keep the friendship, I think it would be reasonable for you to suggest a drink or something and even though it is a family vacation, I think your husband should help you sort this out by being supportive and allowing you a couple of hours.
It is totally reasonable to want to have a family vacation without other people.
I also think it is not unreasonable to tell a friend when and where you are planning a vacation. I'm sure most people here tell their friends about Disney because we all love it so much and get totally pumped about going.
Good luck and enjoy your vacation.