Should there be a parent task "All supported extensions should have an MW-Vagrant rôle"?
That would be a great goal but I'm afraid it would be a reprise of T2001: [DO NOT USE] Documentation is out of date, incomplete (tracking) [superseded by #Documentation] in a slightly different form. I'd rather see each WMF product team commit to using MediaWiki-Vagrant and actively supporting their components like the Multimedia team is here.
Should this be done for each extension? If is so, I can create subtask for each extension which no have vagrant role?