Where are you starting your Camino? From Porto there will be more options because there are more pilgrims/tourists, and therefore more people with various dietary needs requesting meals.
From Lisbon to Porto, options are fewer but it's certainly doable. Traditional Portuguese restaurants don't cater to vegans; menus are typically divided into meat dishes and fish dishes, with no other options. That being said, every meat and fish dish comes with salad, rice, French fries and bread, so you can always have that if nothing else. Sometimes pasta. Usually there's a vegetable soup too. And if you ask, they sometimes have vegetables they can cook for you.
We found it worth carrying something small like a can of vegan tuna or sausages to supplement these Portuguese restaurant meals.
For self-catering, the situation is somewhat better. It's not unusual these days to find hummus, vegan sausages, seitan, tofu, etc. in Portuguese supermarkets. And even in the very worst case, you can always get a can of chickpeas and smash it on bread to make a faux tuna sandwich. We typically have a self-catered picnic lunch and a restaurant dinner.
Check the highlight reels of my Instagram account
www.instagram.com/nomadic_vegan to see exactly what and where we ate each day on various caminos, including from Lisbon to Santiago. Bom Caminho!