Lammas marks when our ancestors celebrated the first fruits of their harvest and prayed for the rest of the growing season to go well. People living close to the earth without connections to outside resources depended on their own harvest for the survival and growth of their communities.
In the story of the Goddess and God, the harvest has a strong connection to sacrifice. The Oak King takes on a new role, in which he is also known as the Corn King. As the corn king, he begins the process of dying so that the creatures of the earth can live. It is a sacrificial death that many young gods throughout world mythology undergo, and of course is reflected in the story of Jesus Christ.
The concept of sacrifice in connection to food and hunger is a fascinating one for the vegan pagan. It is a great time of year to be mindful of the full cost of food, in terms of compassion, environmentalism, justice, etc. As vegans we already spend a lot of time thinking about these interconnections. Lammas is therefore a holiday where we can earnestly celebrate and be grateful for the food that we have. We can also be grateful for the spread of veganism and the availability of cruelty free, ethically obtained items (to a greater and greater extent).
Pagan concepts of wheat or corn (or other plants) as a gift from a deity are a deep metaphor for sustainable food on our earth. This is despite what modern agriculture has done to plants with genetically modified seeds and lack of crop diversity. Plants are a renewable resource that our planet naturally produces if we get out of the way. We can also easily produce crops of plants if we study environmentally sound ways to do so (most of which harken back to practices our pagan ancestors used).
I think Lammas is the perfect time to explore the work of A Well Fed World (www.awellfedworld.org). This organization's work is very connected to the spirit of the Corn King's gift. That there is enough bounty in plant crops to feed people everywhere. If we farm in environmentally responsible ways, everyone should have enough to eat. This practice would also liberate our animal brothers and sisters from our exploitative animal agriculture.