Vampirism has an API which allow the creation of addon mods as well as compatibility integration in other mods.
It is located in the
If some API functionality is missing, create an issue or contact us on discord.
Checkout this example project: https://github.com/TeamLapen/VampirismAPIExample
If you want to create an addon which access all of Vampirism's classes, not just the API, checkout this https://github.com/TeamLapen/VampirismAddonExample and consider contacting us.
A real example is the VampirismIntegrations mod which is some kind of mixtures between the two kinds above: https://github.com/TeamLapen/VampirismIntegrations