Some IdentityReferences cannot be translated to [System.Security.Principal.SecurityIdentifier] because they don't map to a SecurityIdentifier. An example is: IdentityReference : APPLICATION PACKAGE AUTHORITY\ALL APPLICATION PACKAGES In such case, it is better to catch the exception and treat it as $false.
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.