I was in Vigo last year on the Grand.
The city is compact and walkable.
Nice downtown. Good shopping. Clean.
The fort at the top of the highest point is well worth the long walk-up.
The views of the harbor were beautiful.
There's a beautiful garden with senic statures.
After an exhausting walk up all the levels, when we got to the top, we noted that cabs were bringing visitors to the top. (Well worht the cab fare)
You can keep yourself occupied for 4-6 hours
The cruise ships dock well within walking distance of Vigo.
It's a steep walk up to the city proper from the dock.
I found a tourist office on the way up. Helpful maps.
I'd estimate it's about 1/2-1mile up from the ship to the mainstreet.