He can absolutely drain energy from the plants around him, though they are none too happy about this fact! They will try to take big bites of flesh out of him as soon as they feel their energy waning, and the vines will attempt to root him in place and maybe steal and eyeball from him if they can. The plants are mean, but not very strong, so he can beat them back into submission pretty easily.

He will not be able to harvest energy from those in solitary, though; there is some kind of magical barrier/blockade kind of thing preventing him from doing so.

Unfortunately, Vicious's Mystic Arte will not be able to blow up an entrance into solitary confinement. It doesn't even seem to dent the door or the walls around it, which seems very strange! After his attempt, Vicious will begin to feel super fatigued and super sleepy, and he'll find himself passing out on the spot. While blacked out, he'll hear a woman's voice, feel the sensation of a hand touching his cheek tenderly, and hear her speak the words-- They are okay, my child. They are protected.

When he wakes up, all will be as he left it. There will be no sign of a woman anywhere nearby, but he will have a large, green-ish blue snake scale tucked into his palm.