Top-level parameters can only be created for scalars. Vectors are indeed not supported in this context.
Maybe you can add an extra scalar parameter to your model, assign it to the desired vector element inside your model code, and then create a top-level parameter association for this extra scalar parameter?
[ In the upcoming WEST 2017 release, there will be another way around this, i.e. through the new concept of "top-level equations" ]