You’re on the right track by placing the light vector in a parameter collection. You don’t want to replace that with a blueprint: you want to update that vector parameter from your blueprint instead.
Take a look at the skysphere blueprint that comes with the engine, this uses a similar approach to transfer the sunlight direction.
If you want to have your shader influenced by multiple light sources you have to add multiple light vectors and a weighting factor to you parameter collection.
Finally note that this approach won’t really work with shadows at all.