@RWK1952
Depending on the specific port, we do a mix of DIY days, private shorex, and cruise line shorex. Typically, if the destination is quite far from the port, and/or the next port is pretty hard to reach (e.g., the other side of the pond on a transatlantic), we would be more inclined to do a cruise line tour.
The typical problem with cruise line tours is that you are generally on a big bus with about 50 of your new friends. And the quality of the guides varies. I think that private tour operators are less likely to keep a guide who is just not very good.
It terms of picking private tour operators, of course it's critical to do advance research -- e.g., reading up on Trip Advisor and elsewhere, and reading up and asking questions here on CC in the specific port of call forums for your cruise.
Some comments about several of the ports you will be visiting:
Vigo, Spain -- we found the city itself not very interesting; I think the best thing to do here is to leave town and visit Santiago de Compostela, the old town of which is a UNESCO World Heritage Site. The Cathedral is a centuries-old famous destination of Catholic pilgrims, who come on foot and by many other modes of transportation. (See the happy bicyclists in my photo below.) Probably a good opportunity for an NCL tour.
Palma de Mallorca is do-able on your own -- take a shuttle (if one is running) or a cab into the heart of the city, and you can tour the magnificent Cathedral and surrounding area on your own schedule.
In Le Havre, I would strongly recommend taking a private tour of the D-Day sites with the highly regarded Overlord Tours. They were fabulous (and if you head over to the France port of call forum you can get more feedback). They book up quickly. (Paris is a very long way from Le Havre.)
I hope this was helpful. Enjoy your cruise!