Tried to run vagrant ssh-config from within capistrano, and got the following message:
Vagrant experienced a version conflict with some installed plugins! This usually happens if you recently upgraded Vagrant. As part of the upgrade process, some existing plugins are no longer compatible with this version of Vagrant. The recommended way to fix this is to remove your existing plugins and reinstall them one-by-one. To remove all plugins: ...
Turns out, the fix was to run ssh-config like this:
system("unset RUBYLIB; vagrant ssh-config")
Found the solution here