I haven't been a lot to Germany as a vegan (except briefly once), but I have been travelling to lots of other places.
Where specifically in Germany are you going? I'm sure the bigger cities are not complete vegan wastelands.
Obviously, if you're going on your own you can go to eat wherever you want, but if you're with a group it can be more difficult. Check the obvious sites for info about the specific place you are going. Find out which restaurants, cafes etc will have options for you. Find out which supermarkets have the best options.
Where are you staying? Will you have access to a refrigerator? Cooking facilities? If you have a refrigerator, you can buy soya milk or another plant milk for breakfast cereal, so then at least you have breakfast sorted out. If not, you may be able to find small packages of soya milk or soya yoghurt that don't require refrigeration, or that you can go and buy every day for your breakfast.
Also, are you staying over a weekend? Remember that lots of places probably close on Sundays.