Variable interpolation

I am trying to interpolate variables within an Ansible play. I have the following variables:

target_hostname
user_account
account_password

user_account_password is in a vault, and the variable is named target_hostname.user_account.user_account_password, e.g., myhost.username.thepassword.

The variables defined:

build:
  common:
    system_accounts:
      - name: 'ansible'
        group: 'ansible'
        uid: '5000'
        gid: '5000'
      - name: 'sysadm'
        group: 'sysadm'
        uid: '5001'
        gid: '5001'

And the entries in the vault:

target_hostname:
  bootloader_password: bootloader123
  root.account_password: root123
  sysadm.account_password: sysadm123
  ansible.account_password: ansible123

Tried using varying types of interpolation:

- name: Ensure users exist with appropriate UID
  ansible.builtin.user:
    name: "{{ system_account_items.name }}"
    uid: "{{ system_account_items.uid }}"
    umask: "022"
    group: "{{ system_account_items.group }}"
    password: "{{ target_hostname.[{{ system_account_items.name }}].account_password | password_hash('sha512') }}"
    update_password: always
  with_items: "{{ build.common.system_accounts }}"
  loop_control:
    loop_var: system_account_items

Solution

For example,

    - debug:
        msg: |
          name:  "{{ item.name }}"
          uid:   "{{ item.uid }}"
          group: "{{ item.group }}"
          password: "{{ target_hostname[key] }}"
      loop: "{{ build.common.system_accounts }}"
      vars:
        key: "{{ item.name }}.account_password"

gives

ok: [localhost] => (item={'name': 'ansible', 'group': 'ansible', 'uid': 5000, 'gid': 5000}) => 
    msg: |-
        name:  "ansible"
        uid:   "5000"
        group: "ansible"
        password: "ansible123"
ok: [localhost] => (item={'name': 'sysadm', 'group': 'sysadm', 'uid': 5001, 'gid': 5001}) => 
    msg: |-
        name:  "sysadm"
        uid:   "5001"
        group: "sysadm"
        password: "sysadm123"

^{Example of a complete playbook for testing
- hosts: localhost

vars:

build:
common:
system_accounts:
- name: ansible
group: ansible
uid: 5000
gid: 5000
- name: sysadm
group: sysadm
uid: 5001
gid: 5001

target_hostname:
bootloader_password: bootloader123
root.account_password: root123
sysadm.account_password: sysadm123
ansible.account_password: ansible123

tasks:

- debug:
msg: |
name: "{{ item.name }}"
uid: "{{ item.uid }}"
group: "{{ item.group }}"
password: "{{ target_hostname[key] }}"
loop: "{{ build.common.system_accounts }}"
vars:
key: "{{ item.name }}.account_password"}