Hi David. I am vegan and have been living in Portugal for nearly 7 years. There is far, far more vegan food available now than there was when I arrived, both in restaurants and supermarkets.
Of course, it depends where you live - I live in central Portugal and don’t have a huge range of restaurants available, but I know I can eat a vegan meal in a couple of local restaurants. Lisbon and Porto are completely different, there are many vegan restaurants and vegan options in other restaurants. We had a weekend in Porto a couple of months ago and couldn’t believe the choice.
There are lots of things available in supermarkets too, especially in the larger ones. There might not be quite the choice you are used to, but you certainly won’t starve. And, in my experience, people in cafes/restaurants are very happy to make you something as long as you are able to explain what it is you need.
Finally, yes - the law requiring all public institutions to provide a 100% vegetarian (ie vegan) meal is in place. It applies to schools, hospitals, prisons etc, not privately owned places like restaurants.