Will allow you to set variables needed before dynamic inventory is run, allowing you to use ansible vault

Ansible vault with AWS Dynamic Inventory

---
- name: Set env variables
  hosts: localhost
  connection: local
  environment:
    AWS_ACCESS_KEY_ID: "{{ AWS_ACCESS_KEY }}"
    AWS_SECRET_ACCESS_KEY: "{{ AWS_SECRET_KEY }}"
  tasks:
     - name: Build AWS Inventory
       command: "{{ ec2_py_location }} --list --refresh-cache"
       register: aws_servers
     - name: Create fact composed of all the new servers
       set_fact:
         new_aws_servers: "{{ aws_servers.stdout }}"
     - name: Add hosts to in-memory inventory
       add_host:
         name: "{{ item }}"
         groups:
           - aws
       with_items: "{{ new_aws_servers._meta.hostvars }}"

- name: Execute
  hosts: aws
  gather_facts: false
  tasks:
   - name: "Display the AWS servers this play will operate over"
     debug:
       msg: "{{inventory_hostname}}"