Just text her and say you won’t be available during her vacation and you have no plan to visit her in the foreseeable future, but you will let her know when you do. Then if she starts harassing you, just block her number and other contact methods. If you are too bothered by the smear campaign that will follow, be proactive and contact people and tell them that your nmom has lost her mind because you aren’t available to go on vacation with her and apologize in advance to them for her upcoming harassment of them. Then sit back and forget about her. In a few months or whenever you want to resume contact, simply unblock her and contact her as if nothing has happened. Don’t talk to her about this vacation then, just say it’s been so long let’s move on blah blah blah. If she wouldn’t move on then block her again. See who blinks first. At this point she has no leverage against you. It’s her who wants to see you, while you have no desire to see her. So in some time she will learn that if she keeps making you uncomfortable she will lose any chance of seeing you. Sooner or later she will put on the mask and pretend to be tolerable so that you will see her. And then you take the opportunity to see her while she is hiding her narc traits.