Betterloid is an unofficial Plugin system developped for VOCALOID5 Editor.
https://www.youtube.com/watch?v=3JE_36VKGHM
It is fully open source under GPL license and doesn't require any patching or modifications to your VOCALOID5 installation.
To make plugins or to build, refer to the same instructions as the ones from this repository but change the git clone url : https://github.com/SeleDreams/Betterloid.PluginTemplate
To install it, either download a prebuilt version or build it then extract the files in your VOCALOID5 Editor directory and start Betterloid.exe, this is all you have to do, when you want to install a plugin, copy paste the directory of the plugin you want to install in the Plugins folder