I fast on Wednesdays and Fridays most weeks of the year, observe four major fasting seasons (Great Lent, Apostles Fast, Dormition Fast, and Nativity Fast), and also observe several additional fasting days (the Beheading of St. John the Baptist and the Exaltation of the Holy Cross come to mind as examples). Fasting consists in limiting intake of certain kinds of food. The most common form is no meat, eggs, dairy, or vertebrate fish, but there are also other forms.
Are vegan versions of these food types allowed?
Yes, they are. The availability of vegan food alternatives in the Bay Area makes it much easier to observe the fast here than it would be in the South or the Midwest.