Hello!
If you start in Vigo and walk the Coastal Route, next stop will be Redondela - about 15.7km of walking, in my opinion quite doable in one afternoon, especially because it does not seem to have a lot of climbing. You would be able to get a Compostela as it is 102.8km from Santiago.
Alternatively, if you start from Porriño, if I'm not mistaken it would be about 105km more or less, which would also be suitable for the Compostela but oh, boy, you got an afternoon of climbing and going down hill there! I did Tui-Redondela (33km) in one day and I admit, it was hard. We reached Porriño at lunch time and then arrived in Redondela at about 7pm. Now it is up to your abilities.
As mentioned before, Porriño is kind of an industrial city, but upon leaving the city you go between villages and mountain up to Santiaguinho and it is a very beautiful walk.
Buen Camino and have fun!