I've just walked the Camino Francés from St Jean Pied de Port as a vegan, and my experience was extremely positive. On most days I self-catered with bread, fresh produce and snacks for my mid-day meal while walking, but in the evenings I mostly ate hot meals at restaurants or albergues, and I found these places to be very accommodating.
I was surprised at just how many albergues served only vegetarian (in reality mostly vegan) food. And those that served meat were usually able to prepare a vegan option on request. Contrary to some statements made by other (non-vegan) posters above, my vegan lifestyle did not limit my socializing on the Camino in any way. I ate about a dozen communal dinners in albergues with other pilgrims. In 39 days, my husband and I cooked dinner for ourselves only five times, and it was never because that was the only option. And of course, on those days we still socialized with other pilgrims in the hostel kitchen.
Disclaimer: I speak Spanish pretty fluently, have been to Spain 10 to 12 times before and am very familiar with Spanish cuisine, so I know which dishes on the menu are likely to be vegan or veganizable. For pilgrims without this background knowledge, eating out will be more challenging. In an effort to help other pilgrims overcome this obstacle, I have written a blog post with my
top tips for eating vegan on the Camino. It includes a downloadable cheat sheet for ordering vegan food in Spanish on the Camino.