You are trying to parallelize this loop:
for idx in axes(random_vehicle_pairs, 2)
but you have:
size(random_vehicle_pairs) = (2, 5)
and each iteration of the loop is fast. So it is very unlikely that this parallelization will help at all, unless your “real” examples have a much greater number of loop iterations. And, in that case, you should try the parallelization with the real numbers.