Travelling when you are vegan can be a little difficult, but if you do some work before you go then it should be plain sailing.
Research as Much as You Can
Look into where you are going, not just for fun things to do, eat and see, but for things to avoid. I went to Seville last week and whilst I had an AMAZING time I knew I did not want to go anywhere near their bullring so I made sure it wasn't beside anything I wanted to do.
Plan Plan Plan
Looking for places to eat can be annoying especially if you are hungry and everywhere just seems to have ham & cheese in every dish. I like to go onto happy cow (they have an app but the website is free) and bookmark any restaurants that look interesting. I'll then check reviews and then their location to any tourist spots I'll be visiting! That way I don't have to plan in loads of travel time for food breaks when I want to explore as much as possible.
Learn the Language
This can be something as simple as learning how to say hello, how are you! You can learn how to say you are vegan or that you can't eat meat or dairy, or just that you can only speak English! Trying a little goes a long way.
Give them Options
If you arrive somewhere that's got no vegan options on the menu and there isn't anything that can be substituted then give the waiter some help. Ask if they can do some pasta and roasted veg, or even if you can make a vegan starter into the main course! If you are in Europe be aware that most nice Italian places make their pasta from scratch, this means it'll contain eggs. BUT, if you ask for gluten free pasta it's usually dried & egg free! Just remember to double check bases, sauces and dressings, just because something doesn't say milk/cream/meat doesn't mean it's not there.
Don't Get Frustrated
If a waiter doesn't understand what you are asking don't get frustrated. Some places are still learning what veganism is and helping them instead of getting mad will not only ensure they are willing to help you find something to eat, it'll also mean that they will have a clear understanding when the next vegan walks through their door!
Finally, remember that it's ok to slip up! We all make mistakes and trying your hardest is ALL you can do. You are on holiday, you should be relaxing and enjoying yourself and NOT beating yourself up about something you ate without knowing.