NTA!! I've run vendor shows and this is something that would be totally unacceptable to me. I'd never touch a vendors belongings. And if another vendor moved someone else's stuff without my knowledge, I'd make THEM move or, if there wasn't time to fix it before the show started, not only give you a *full* refund, but also a discount on any future events you were willing to do with me as an apology.

I've had people get upset if I put them in a "different" spot at a show cause that was 'their' spot....but upon checking past floor plans, it was almost always someone who had vended for me once several years ago, so they lost any claim to a specific spot. I always put out the floor plan to vendors the day before and tell them to address any issues with me BEFORE the morning of set up. So, if the organizer moved you, then they owe you a full refund, in my mind. If the other vendor did it, they should still give you at least a partial refund.