Al it needs is a bit of research. If you use a browser and type in the words 'message and cruise and europe' (no need to put in the 'and') - of the results returned you need approx 3rd link down that begins with the word Europe. Make sure you click on 'all results' and not just ones from the UK if you use the Goog site.
While the tours available from P&O are at a cost it is important to bear in mind that if the ship docks for a morning and you go off independently and come unstuck it is you who will have to find your own way to the shipos next port of call. Having said that the tour we did last Nov was a very interesting one but having done it I would not want to repeat it.
Its around 50miles from Vigo to Santiago so taxi would be quite expensive. Trains are available, you can check out times and fares here http://horarios.renfe.es/hir/ingles.html?