Try using:
'orderby'=>'distribution_list last_name',
'order'=>'ASC'
Instead of:
'orderby' => array(
'distribution_list' => 'ASC',
'last_name' => 'ASC',
),
So:
$users = new WP_User_Query( array(
'meta_query' => array(
'relation' => 'AND',
'lastname' => array(
'key' => 'last_name',
'compare' => 'EXISTS',
'type' => 'CHAR',
),
'list' => array(
'distribution_list' => array(
'key' => 'distribution_list',
'value' => '"72"',
'compare' => 'LIKE'
),
'distribution_list2' => array(
'key' => 'distribution_list',
'value' => '"72"',
'compare' => 'NOT LIKE'
),
'relation' => 'OR',
),
),
'orderby'=>'distribution_list last_name',
'order'=>'ASC'
) );