Trying to get vault and vars to work in templates with Ansible.... colons in file names represent directories

- output of ansible playbook run when it was broken -

fatal: [ge-spark-master1] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'vaulted_access_key' is undefined", 'failed': True}
fatal: [ge-spark-master1] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'vaulted_access_key' is undefined", 'failed': True}
fatal: [ge-spark-master2] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'vaulted_access_key' is undefined", 'failed': True}
fatal: [ge-spark-master2] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'vaulted_access_key' is undefined", 'failed': True}

roles:spark_common:tasks:main.yml

---
# file: roles/spark/tasks/main.yml

- include_vars: vault.yml
  tags:                     # These were the lines I was missing
    - spark                 # These were the lines I was missing
    - spark-masters         # These were the lines I was missing
    - spark-workers         # These were the lines I was missing

- include_vars: unencrypted.yml
  tags:                     # These were the lines I was missing
    - spark                 # These were the lines I was missing
    - spark-masters         # These were the lines I was missing
    - spark-workers         # These were the lines I was missing


- debug: var=vaulted_access_key

- name: Copy stuff with vars
  template: src=site.xml dest={{ spark.location }}/conf/site.xml
  tags:
    - spark
    - spark-masters
    - spark-workers

- debug: var=vaulted_gcs_access_key

- name: Copy stuff without vars
  template: src={{ item }} dest={{ spark.location }}/conf/{{ item }}
  with_items:
    - spark-env.sh
    - spark.properties
    - log4j.properties
    - metrics.properties
  tags:
    - spark
    - spark-masters

    - spark-workers

roles:spark_common:templates:site.xml

 ...
 <property>
  <name>access.key</name>
  <value>{{ access_key }}</value>
 </property>

 <property>
  <name>secret.key</name>
  <value>{{ secret_key }}</value>
 </property>
...

roles:spark_common:vars:unencrypted.yml

access_key: "{{ vaulted_access_key }}"
secret_key: "{{ vaulted_secret_key }}"

roles:spark_common:vars:vault.yml

vaulted_access_key: ACTUALACCESSKEY
vaulted_secret_key: ACTUALSECRETKEY

sanity check grep output

$ grep -irI vaulted_access_key
roles/spark_common/tasks/main.yml:- debug: var=vaulted_access_key
roles/spark_common/tasks/main.yml:- debug: var=vaulted_access_key
roles/spark_common/templates/core-site.xml:  <value>{{ vaulted_access_key }}</value>
roles/spark_common/vars/vault.yml:vaulted_access_key: ACTUALACCESSKEY
roles/spark_common/vars/unencypted.yml:access_key: "{{ vaulted_access_key }}"

you probably need to use :
vars_files: vault.yml
instead of include_vars.
Or is this working for you?
Unclear what you mean otherwise.