I would like to understand at what point the vectorisation stops to give performance advantage.

Some examples:

- This clearly gives great speedup

`int[N] ~ neg_binomial(real, real);`

- What about this? (where real[N] is some tranformed parameter of a <N parameter space)

`int[N] ~ neg_binomial(real[N], real);`

- What about this? (where real[N] is some tranformed parameter of a <N parameter space)

`int[N] ~ neg_binomial(real[N], real[N]);`