The walk along the coast between River Douro and Vila do Conde can be tiring at places because the boardwalk is broken and you have to walk on sand (not long distances, anyway), and other times you will have to walk in land. There are some confusing bits, like when you pass Angeiras, you find arrows sending you inland, but after a while you stop seeing arrows. Also, the arrival in Vila do Conde is via tarmac.
I would avoid walking from Vila do Conde to San Pedro de Rates, it just tarmac and very tiring cobblestones (bar the last 800 meters), the traffic is atrocious at places, and there are no hard shoulders. This is a part to be avoided, in my opinion.
I just did that part about one month ago, if I had to do it again, I would walk down the river and then up the coast in Matosinhos, then take a bus or train to San Pedro de Rates or even Barcelos in order to avoid tarmac, traffic and cobblestones. Barcelos is a gorgeous town