This is just a convenience enhancement - rather than swallowing all the output from the Heroku push command, print stdout and stderr like we do elsewhere with subprocesses. Heoku pushes are pretty involved and dump lots of useful output, so it should be useful for users to see this.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.