The jump over to the central route is not the easiest at Viana do Castelo.
I did it before and will it do next month .
Before Viana in Castelo do Neiva or Chafé you could find your way to a close by trainstation and take the train to Durrães.From there you walk to the village of Balugães where you find the yellow waymarkers of the central route to Ponte de Lima.
Lucky you are because three kms after Balugães you'll find the best albergue of the entire caminho Português,
Casa da Fernanda in the hamlet of Vitorino dos Piães.
Give Fernanda a call on beforehand. It can be busy there so reserve your bed.
+351 914 589 521
Walking from Viana to Fernanda is an option but it is about 27 kms and a lot walking along a national road.
You can continue the coastal to Caminha and there follow the river Minho -at your left to Vila Nova de Cerveira and Valença do Minho which is only a throw of a stone away from Tui. After crossing a bridge in Caminha you follow a fairly new foot-and cyclepath, the Ecovia to Valença.
Desavantage is that you'll miss
Casa da Fernanda.