I stop by the Seaport Market almost every weekend. I buy some of my produce there and I usually get something to eat.
One thing I like about the Seaport Market is that it is full of surprises. There is so much there that you'll miss things if you only go once. I have been going for over a year and I just recently discovered a Turkish booth selling vegan items that have 5 ingredients or less and the German bakery that sells delicious vegan, gluten-free bread.
My favorite vegan vendors include Fruition (raw foods) and The Kind Cookie (vegan, gluten-free baked goods).
Saturday is the day to go. Most of the vendors are there. Saturday is also the busiest day. It's hard to find parking at the market, so we always have to park in pay parking across the street.Pros: many vegan vendors
Cons: very busy on Saturdays, no parking