Full disclosure: not vegan, but have lived with some vegans in Europe. Depending on where you are touring there, it can be very difficult. Food labeling laws aren't always adequate, even if you know the local language to begin with. Obviously if you are preparing all your own food, you'll find much of what you want in the grocery stores, but trying to find a vegan restaurant somewhere like rural Czech is simply not going to happen.
Best advice I can give from them, if you aren't strict vegan because of innate ethical beliefs or medical issues, be open to some compromise. Even as a non-vegan, I live by that whenever traveling outside of home.
Also, depending on where you are considering, it is worth checking out India for a trip. It is dead easy to be at least completely vegetarian there (can't speak to whether their cooking practices are vegan or not).