De Bolhoed is a pleasant vegetarian restaurant on Prinsengracht, one of the closest vegetarian sit down restaurants to the center of the city. It has a neighborhood restaurant flavor, and while the service can be slow by American standards (not really a point against it since this is true of almost ANY dutch restaurant), I always have a pleasant meal there.
The vegan daily special (which, unlike the other reviewer, I've never had the misfortune to learn had run out; we usually come around 6 though) is always a huge plate of food, usually including a salad, a couple of veggie sides, and a few ethnic specialties. One day it was classic Dutch food, great to have the opportunity to try vegan; I loved my kale stamppot (a mashed potato-ish dish), although the veggie burger patties were a bit too...whole grain...for me. The next time it was thai, with curries and rice dishes. Usually some parts of the daily special are mediocre, others are incredibly tasty. The presentation is a little homey - expect to get a plate overflowing with different dishes crammed together on your plate - but there's always something deliciously tasty on it.
The desserts are some of the best vegan desserts I've had in Amsterdam, which unfortunately isn't saying that much, but they're not bad. In general the vegan cuisine has a ways to come in Holland, but here the cuisine is something like good home cooking with a completely unrushed atmosphere. There's no idea here that the servers need to turn over n tables an hour. Don't come unless you've got a few hours to savor conversation and company with your dining companions, but for those times when you do want to linger and catch up, it's great.