"sed -i" is specific to GNU sed and is not a posix standard option. The default sed command in Solaris is not GNU sed though it is possible to install the GNU-sed which includes /usr/bin/gsed
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.