Why do COVID19 serology tests focus on the nucleocapsid while vaccine efforts focus on the spike?
The vaccines will be directed towards a protein/epitope of the virus that creates the most robust immunogenic response, in which the defense system will create an array of antibodies to confer immunity, those antibodies will have to NEUTRALIZE the virus upon entry. 
In the case of SARS-CoV-2, the majority of research shows that neutralizing antibodies are directed towards the S1, S2 and RBD proteins which are all part of the S protein. 

In this same line, the fact that the body creates antibodies directed towards other components of the virus doesn't mean that they are protective. So creating a vaccine towards those components wouldn't be useful at all. 

The reason why those serologic tests target the N protein is probably due to the fact that is highly antigenic. But I'm sure I've seen tests that also target the S protein.