As a vegan living in Portugal, I can tell you that Porto has some incredible veggie dining options. Even smaller cities and towns in Portugal will often have one vegetarian restaurant (usually buffet or canteen style). Traditional Portuguese cuisine, however, is not at all veg-friendly. Most restaurant menus are divided into two sections: meat dishes and fish dishes.
Portuguese people are very friendly and accommodating, though, and will generally be happy to make something for you that's not on the menu. This will often be rice and vegetables. Portuguese cuisine does use chickpeas, beans and other legumes, so these might be an option if they haven't been cooked in a meat-based broth. If worse comes to worst, there's always salad, French fries and sopa de legumes (vegetable soup). Actually, many Portuguese soups are vegetarian, or can be. Again, ask about the broth if that's important to you.
In Spain, you have more options. Gazpacho, paella de verduras and patatas bravas are a few examples of traditional vegetarian dishes that are very common in Spain. On my blog I've written a guide to eating vegan on the Camino, which you'll find linked in my signature below. It's based on my experience walking the Camino Francés and the Camino Primitivo, but the tips would equally apply on the Camino Portugués. Also take a look at my ultimate vegan guide to Spain while you're there.
Bom Caminho!