I just saw you've walked the Frances, so this is probably your second Camino. The Portugues was my second one as well. If you have to choose, I would probably recommend the Porto to Santiago, and then continue up to Fisterra and Muxia (but despite what the guidebooks say, I would, when leaving Pirto, walk early in the morning along the river in Porto to the coast, rather than taking the tram there). After Porto, you a beautiful coastline, some pretty hills and it's much, much easier on the feet than the Lisboa to Porto leg. There are many more people, as well, though I wouldn't say, when I walked it, that it's overcrowded. You also have the option of walking along inland or along more of tvhe coast on this part of the Portugues.
Of course, it really depends on what you like. Lisboa to Tomar is pretty flat, and can get pretty hot in summer, and most of the way up to Porto is very hard on the feet (lots and lots of cobblestones and asphalt), but you walk along the River Tejo a lot - but not all - of the way for the first few days, which is nice, and I really liked Tomar - a beautiful Templar town. Past Tomar all the way to Porto it still is pretty tough on the feet, but you get a taste of "off the beaten path" Portugal, and you eventually get to pass through Coimbra, which is a very lovely city. And again, you get to stay at some very nice albergues on the way, though the first eight days or so of the Lisboa to Porto walk involve very (long 30 km and more) stages. On the other hand, I met no more than six or so folks the whole way to Porto, and some of them ended up taking buses to Porto! As I mentioned, after about the eighth day, I was ready to meet other walkers, even though I am pretty independent - but that didn't really happen until I reached Porto
from there to Santiago, and beyond, that's not a problem at all.
One other thing worth mentioning... One advantage of starting in Lisboa is Lisboa and environs itself. It is a gorgeous city and the nearby coast - though not part of the Caminho - is absolutely amazing, although you can always bus or train it back when you finish your walk if you do the Porto to Santiago leg.
As a fellow vegetarian - and an "almost" vegan (the very occasional yogurt in a pinch, but no eggs or cheese), hmmm, I have to say, food choices were kinda slim - I stocked up as much as possible (avocados and nuts as much as I could) at the larger supermarkets - salads and bread at night, though in Portugal there's a very delicious bean soup which is vegetarian as well. But food was problematic for me on all three caminos I've walked - whether in Portugal or in Spain - I am a bread lover but by the end of each walk I was ready to swear off bread for good!
. (Of course, that didn't last very long
) At least the bread is pretty good in Portugal and in Galicia - usually!
Finally, I can't recommend strongly enough to continue on - weather be damned! - to Fisterra and then to Muxia. It is definitely worth it! And if you're lucky, you might not get wind and rain to boot!