I'm so glad to hear that the in laws weren't totally intrusive. Its unfortunate that your husband felt the need to tell a fib so he could keep his mom happy and in the loop. I think before you plan another vacation, you need to have the rules set in stone.
I have in laws that will "show up" if they know we are going somewhere. We have learned to keep mum about vacation and holiday plans when it comes to family. We have gone as far as to lie to them about holiday plans just so they don't invite themselves. Maybe you need to get that sort of agreement settled with your hubby for future outings.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France