We stopped at Vigo on our recent Regal Princess cruise. Some of our tablemates took a taxi over to Santiago de Compostela. They got it at the dock (you're essentially right downtown). We were only in port for 5 hrs and he drove like a madman to get them there and back in time, but they thought it was well worth it.
I actually did this by train last summer. We are fast walkers, and we actually managed to walk to the train station, grab the train to Santiago, walk to the church (we were probably there for 2 hours), grab the train back and stop in Pontevedra for a couple of hours too. My memory is we were back on board by about 5? Check the train schedules online to see if it would make sense for you.
While Santiago was impressive, I have to say I enjoyed wandering around sleepy Pontevedra just as much...
I will be in Vigo September too, and want to do the same. My research indicates the Castomil autobuses are more frequent. The bus station is a bit further away, but my estimate is that I can do both Santiago and Pontevedra in about ten hours, and definitely Santiago can be done independently by train or bus in a day.
Enjoyed your comments about Vigo, and your English was more than adequate. Looked up your home page and the fotos you referenced, and they were all great, especially the one of the cruise ship centered in the rainbow. Vigo appears to be a very interesting city, and we plan to spend our time there just enjoying the city. Thanks for the helpful information.