group_vars/ should reside in inventory directory.

[kaaquist]

Thanks for a really easy to read example of AWS with Ansible.
I just had a few problems with you code using Ansible 2.5.2.
The group_vars/ now need to be located next to the hosts file. Therefore it needs to be moved to the inventory directory.
Plus for the group_id, to work you need to include a vpc_subnet_id too in the ec2: -> ansible/ansible#29519