You can walk to the town in Vigo. It is uphill but easier if you turn left after security rather than right which would take you up steps and through a lot of fish restaurants. The main street has lots of big name shops like Zara, Sephora etc if that is what you are looking for. There is a small shopping mall by the port but it mostly sells electricals and booze and some clothes.
In Lisbon you catch a tram into the main square and there you can catch the hop on hop off bus. There is a tourist information office on the opposite side of the square to where the bus leaves. You buy a ticket from a stationary bus and then queue for the 'moving' one. If the queue is long get one person to wait in the line while another buys the tickets.
Can't help with the other ports as we didn't do them. Never been in April but went to Vigo once in November and it was shorts and t shirt weather.
As said you are virtually in the town centre in Vigo. In Lisbon there is a free shuttle bus into the main square in the city. Weather could be quite hot in Lisbon, slightly cooler in Vigo but as everywhere chance of rain.