Hi there!
I believe if you try to put my plugin somewhere below the Visustella plugin, maybe it works. What happens is that I overwrite a default funcion that shows the Exp gain after the battle. So without Visustella, the plugin will show the exp properly like you said.
But I don't know how Visustella shows the Exp. Maybe it does not use the default functions at all. If they use, maybe put my plugin after them on the plugin manager may solve. If not, try to check if they have an option on the plugin parameter to set where they get the exp gain. If they do, maybe we can find a way through there.
Now, all my plugins need the Eli Book. And if you are not using it, besides the error in game, the RPG Maker plugin manager window will show you a warning on the bottom of the window:
The thing is, RPG Maker will only show one warning at the time. So if you already have a warning there, it will not show you the one from my plugin saying that you need Eli Book.