vlib implements a straightforward plug-in DLL mechanism. VLIB client applications specify a directory to search for plug-in .DLLs, and a name filter to apply (if desired). VLIB needs to load plug-ins very early.
Once loaded, the plug-in DLL mechanism uses dlsym to find and verify a vlib_plugin_registration data structure in the newly-loaded plug-in.
For more on plugins please refer to Adding a plugin.