Vilnius is the capital of Lithuania. It is fairly easy to be a vegan here: even though many still won't know what veganism is, you have a variety of vegan-friendly shops and cafés to choose from. Besides, you will have no problem finding plenty of various fruit&veg in ordinary grocery stores.
In general, there is a feeling that veganism is on the rise in Vilnius. Just 10 years or so ago, it was hard to find a vegetarian place in Vilnius - now there is plenty to chose from, including ordinary restaurants which started labeling vegetarian or even vegan dishes.
Kur valgo veganai? has a listing of vegan, vegetarian and veg-friendly places in Vilnius (in Lithuanian). You can also see Happy Cow's list of vegan/vegetarian places in Vilnius (in English).