Vim is the life force of the Toad species. When extracted from Toads, Vim resembles a green liquid with transparent images of sad Toad spirits. The Toad whose vim is drained will lose their coloration and appear to slowly become part of the trees of Toadwood Forest.
Vim has only ever been referred to in Mario & Luigi: Partners in Time, where a species of alien fungi known as the Shroobs harvested the vim from the Toads to convert into fuel so they can power up their Saucers.
Mario & Luigi: Partners in Time
In Mario & Luigi: Partners in Time, the extraterrestrial Shroobs conquered the Mushroom Kingdom of the past, and abducted many Toads so that their vim could be extracted and converted into fuel for their flying saucers.
Captured Toads were taken to the Toadwood Forest and attached to special bio-engineered trees. These trees could actually suck the vim out of Toads. The vim would then travel through the roots of the trees to the Vim Factory located in the middle of the forest. A giant tree sat in the middle of the factory, absorbing the vim and sending the solution through pipes that led to the lair of Swiggler, a bio-mechanical robot Wiggler that had the ability to convert vim into fuel for Shroob Saucers with special chemicals contained within its body. Flying saucers would attach to Swiggler, which would then transfer the fuel into the ships.
When Mario, Luigi, and the babies first visit Toadwood Forest, Toadiko is being drained of her vim and talks very weak. She tells them the story of what she, Princess Peach, and Toadbert experienced when they landed in the past with Professor E. Gadd's time machine. She manages to tell her tale before Shroob saucers begin shooting purple beams at the brothers, one hitting Toadiko and turning her into a purple mushroom.
Mario, Luigi, Baby Mario, and Baby Luigi put an end to the operation by destroying the Shroobs, the giant vim-sucking tree, and the Swiggler inside the Vim Factory. It was eventually discovered that baby tears reverted the damage done by the Shroobs, and Toadiko is restored to her former self. It is assumed that the tears restored the life force to all the victimized Toads, although it is not confirmed.