Create and manipulate vectors in Stan

The following is somewhat pseudocode of what I want to achieve in Stan. How can I make this work?
One of the problems involves the fact that for the second line, it seems like Stan isn’t liking the fact that I’m using vector instead of row_vector.

vector[27] no_shrinkage_idx = [1:27];
vector[4] shrinkage_idx = [20, 22, 25, 26];
vector[23] no_shrinkage_idx = no_shrinkage_idx[!(no_shrinkage_idx %in% shrinkage_idx)];