So, helpfully, when you install vim-vimoutliner on Ubuntu, per Debian policy, it doesn’t actually work. Unfortunately, following the instructions in /usr/share/doc/vim-vimoutliner/README.Debian
vim-addons install ADDON
doesn’t help, ever after replacing ADDON with vimoutliner (not vim-vimoutliner, which is the package name). In particular, none of the vimoutliner commands appear to work at all and the folding commands (zo/zc) fail to recognise the OTL folds, instead reporting “E490: No fold found”. Your .vimrc must also contain (or your /etc/vim/vimrc must have uncommented):
filetype plugin indent on
1: The reason for this is not obvious; if there’s a problem with potential conflicts then, like all other conflicts in the Debian universe, they should be reflected in the .deb. If there’s a concern about the extraordinarily unusual case where a Debian/Ubuntu machine actually has multiple users and those users actually have conflicting preferences about which vim addons to enable then this should be special-cased. It would seem that a better approach would be to (a) have all addons depend upon vim-addons (because they’re useless until enabled via vim-addons) and (b) have a “default” option called by all post-installers (vim-addons default $MODULE) and an option for the vim-addons command to treat “default” as disabled.
2: Why is this commented out? What is the situation in which you’d want addons that you’d (a) installed and (b) explicitly enabled with vim-addons to still not work?