I've done this for other things, but not an addon template (that I can recall).
I'm trying to override the template file:
design/backend/mail/templates/addons/vendor_plans/companies/common/plan_details.tpl
I'm using an override path of:
design/backend/mail/templates/addons/myaddoname/overrides/addons/vendor_plans/companies/common/plan_details.tpl
I've cleared the registry and template caches via &cc&ctpl but the override template is not being seen.
Anyone see where I'm astray?