UndefVarError: Distributions not defined

[oheil]

You need
using Distributions
inside your module, before importing the function:
e.g.:

module mod_vicsek_model
using Distributions
...
function set_neighbour_orientation(param,var)
         ...
         vm = VonMises(κ)
         ...
     end
 end
...
end

Than you can just call

VonMises()

without Distributions. at front.
(edited to fit more your given information)

https://docs.julialang.org/en/v1/manual/modules/