Copy and paste your template in Dev Tools/templates and you will see what you get.
BTW, your missing a ‘%’ on the end of the first line.
EDIT, and a closing bracket too.
figured out the following, however when I put it into the config.yaml I keep getting an error;
below is the script
{% if states.sensor.plex_vintnet_stats.attributes['andrew.vint']['Activity'] is defined %}
Idle
{% else %}
{{states.sensor.plex_vintnet_stats.attributes['andrew.vint']['full_title']}}
{% endif %}
Below is how it appears in the config.yaml file
value_template: >-
{% if states.sensor.plex_vintnet_stats.attributes['andrew.vint']['Activity'] is defined %}
Idle
{% else %}
{{states.sensor.plex_vintnet_stats.attributes['andrew.vint']['full_title']}}
{% endif %}
Below is the error message when I run the Check Config
Error loading /config/configuration.yaml: while scanning for the next token
found character ‘\t’ that cannot start any token
in “/config/configuration.yaml”, line 649, column 1
This means you have tabs in your yaml.
Yaml should only intended with spaces.
{% if states.sensor.plex_vintnet_stats.attributes['andrew.vint']['Activity'] is defined %}
Idle
{% else %}
{{states.sensor.plex_vintnet_stats.attributes['andrew.vint']['full_title']}}
{% endif %}