Experimenting with trying to use delegate_to to farm out multiple tasks to the same remote host using ansible forks behaviour.

hosts_build

[vagrant]
build

[parallel]
build1
build2
build3
build4
build5

include-parallel-step.yml

---
- name: get the hostname
  command: hostname
  register: hostname

- debug: msg="hello from {{ hostname.stdout }}"

- name: debug service name
  debug: var=service

test-parallel.yml

- hosts: localhost
  tasks:
    - name: setup parallel workers
      add_host: name=task{{ item.0 }} groups=parallel service={{ item.1 }}
      with_indexed_items:
          - one
          - two
          - three
          - four
          - five
          - six
          - seven
          - eight
          - nine
          - ten

- hosts: parallel
  gather_facts: false
  connection: local
  # without this, ansible still attempts to connect to the taskX host to run the
  # 'get the hostname' task even though it's been told to delegate_to the localhost
  tasks:
    - name: build step
      delegate_to: localhost
      include: include-parallel-step.yml

zzz output

PLAY [localhost] ************************************************************** 

GATHERING FACTS *************************************************************** 
ok: [localhost]

TASK: [setup parallel workers] ************************************************ 
ok: [localhost] => (item=(0, 'one'))
ok: [localhost] => (item=(1, 'two'))
ok: [localhost] => (item=(2, 'three'))
ok: [localhost] => (item=(3, 'four'))
ok: [localhost] => (item=(4, 'five'))
ok: [localhost] => (item=(5, 'six'))
ok: [localhost] => (item=(6, 'seven'))
ok: [localhost] => (item=(7, 'eight'))
ok: [localhost] => (item=(8, 'nine'))
ok: [localhost] => (item=(9, 'ten'))

PLAY [parallel] *************************************************************** 

TASK: [get the hostname] ****************************************************** 
fatal: [build1] => SSH Error: ssh: Could not resolve hostname build1: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [build3] => SSH Error: ssh: Could not resolve hostname build3: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [build4] => SSH Error: ssh: Could not resolve hostname build4: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [build2] => SSH Error: ssh: Could not resolve hostname build2: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [build5] => SSH Error: ssh: Could not resolve hostname build5: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task0] => SSH Error: ssh: Could not resolve hostname task0: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task3] => SSH Error: ssh: Could not resolve hostname task3: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task2] => SSH Error: ssh: Could not resolve hostname task2: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task1] => SSH Error: ssh: Could not resolve hostname task1: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task4] => SSH Error: ssh: Could not resolve hostname task4: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task7] => SSH Error: ssh: Could not resolve hostname task7: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task8] => SSH Error: ssh: Could not resolve hostname task8: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task5] => SSH Error: ssh: Could not resolve hostname task5: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task6] => SSH Error: ssh: Could not resolve hostname task6: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.
fatal: [task9] => SSH Error: ssh: Could not resolve hostname task9: Name or service not known
It is sometimes useful to re-run the command using -vvvv, which prints SSH debug output to help diagnose the issue.

FATAL: all hosts have already failed -- aborting

PLAY RECAP ******************************************************************** 
get the hostname -------------------------------------------------------- 6.46s
setup parallel workers -------------------------------------------------- 0.03s
-------------------------------------------------------------------------------
Total: ------------------------------------------------------------------ 7.31s
           to retry, use: --limit @xxxxxxxxxx/test-parallel2.retry

build1                     : ok=0    changed=0    unreachable=1    failed=0   
build2                     : ok=0    changed=0    unreachable=1    failed=0   
build3                     : ok=0    changed=0    unreachable=1    failed=0   
build4                     : ok=0    changed=0    unreachable=1    failed=0   
build5                     : ok=0    changed=0    unreachable=1    failed=0   
localhost                  : ok=2    changed=0    unreachable=0    failed=0   
task0                      : ok=0    changed=0    unreachable=1    failed=0   
task1                      : ok=0    changed=0    unreachable=1    failed=0   
task2                      : ok=0    changed=0    unreachable=1    failed=0   
task3                      : ok=0    changed=0    unreachable=1    failed=0   
task4                      : ok=0    changed=0    unreachable=1    failed=0   
task5                      : ok=0    changed=0    unreachable=1    failed=0   
task6                      : ok=0    changed=0    unreachable=1    failed=0   
task7                      : ok=0    changed=0    unreachable=1    failed=0   
task8                      : ok=0    changed=0    unreachable=1    failed=0   
task9                      : ok=0    changed=0    unreachable=1    failed=0

Since I really just want to delegate to a vagrant 'build' host the following change to the test-parallel.yml works in my case:

- hosts: localhost
  tasks:
    - name: setup parallel hosts
      add_host:
        name: task{{ item.0 }}
        groups: parallel
        service: "{{ item.1 }}"
        ansible_ssh_host: "{{ hostvars.build.ansible_ssh_host }}"
        ansible_ssh_private_key_file: "{{ hostvars.build.ansible_ssh_private_key_file }}"
        ansible_ssh_user: "{{ hostvars.build.ansible_ssh_user }}"
        ansible_ssh_port: "{{ hostvars.build.ansible_ssh_port }}"
      with_indexed_items:
          - one
          - two
          - three
          - four
          - five
          - six
          - seven
          - eight
          - nine
          - ten

- hosts: parallel
  gather_facts: false
  tasks:
    - name: build step
      include: include-parallel-step.yml