At least two companies provide connections from Viana to PdL and vice-versa: Transcunha and A.V. Minho.
They are fairly frequent, see
AVMinho (scroll to the Viana <> PdLima horário).
AVCura serves the coastal towns between Viana and Caminha, also with frequent buses.
I find no english version of the sites, but google translator should automatically do it for you.
Of course, during weekends and holidays services are reduced.
I wouldn't expect them to be very expensive, I couldn't find prices, but those as well as the full list of services available should be clear at the bus stations.
As to the suggestion of walking, the walk is very easy and pleasant if you take the southern side of the Lima river. There are tracks by river virtually all the way, including a waymarked eco-route (unpaved tracks for shared pedestrian and cycle use). Between Darque and the river beach of São Simão it isn't obvious, but if you stay closest to the river whenever possible the route around the marshes and the highway bridge presents itself.
The landscape along the river is beautiful with river, fields and mountains on view. The track is very flat and unless its muddy from the rain it's a delight to walk on.
I believe there was a bar near Passagem (circa half-way, closer to PdL), but aside from this there are no amenities, so you should prepare accordingly.
All the best.
