We have eaten at Terra and Os Tibetanos on a weekend away. Whilst walking from our apartment just off Avenida da Liberdade to both places, we passed several vegetarian restaurants, all of which had at least one vegan option.
Os Tibetanos - nice decor/atmosphere, the food was good but not great, although my omni friend had seitan dumplings and was pretty impressed, having eaten the meat version in Nepal.
Terra - large, all-you-can-eat buffet about 75% of which was vegan. Nothing is labelled, but our waiter was able to point out all the non-vegan dishes. I don't think you can order from the menu in the evenings, it is just the buffet. As many comments have said, the service was a bit grumpy but the general atmosphere was nice and the buffet dishes ranged from good to wonderful. Their website has some great info on being a vegetarian/vegan tourist in Portugal and also a vegetarian dictionary, a sheet you can print off which translates your dietary requirements in Portuguese, something I would highly recommend for travelling outside Lisbon.
It's worth remembering that being vegan is virtually unheard of in Portugal, outside a few restaurants in the bigger cities. Probably 80% of Portugal's vegetarian restaurants would be in Lisbon. A survey commissioned by one of the vegetarian organisations a few years back found that only about 30,000 people identified as being vegetarian (in a population of 10 million!). Most people won't know what vegan (vegano in Portuguese) means and you won't find much labelled as vegan/vegetarian in supermarkets or restaurants outside specifically vegetarian places. On the other hand, if you are able to explain what you want (using the Terra dictionary for instance) you are likely to find that anywhere will be able to come up with something for you, the Portuguese are incredibly friendly and accommodating people even if they think you are completely nuts!