Hey Leanne,
I understand your situation, the only advice i can give you: tell them upfront!If you are in contact with them now, tell them now! I learned one thing and that is that the expection of people should be managed well, and if so, most things are not a big deal. It gets a big deal because of people having other expections and than get disappointed because they are not cming true.
Just tell them asap, in a casual matter. Something like: hey guys, i am so happy i will be staying with you. It is so much nicer than in a hotel. Did i tell you something i think you should know; I am currently eating a plant beased diet. This is not going to be difficult for you as i will bring some of my needs. Also I would love to cook for you this and this blabla...
The longer you wait in telling them, the more difficult it will get, and maybe they will buy extra foods for you, like that special cheese and than it is presented to you and.... you get the picture :)
Good luck and have a wonderful time!
Ps since you metion you travel a lot, check out this site, i think it is great!
http://veganbackpacker.com/category/places/europe/the-netherlands/
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