The Vanishing Twin familiar will return when Isaac reaches the next floor.
If Vanishing Twin becomes a clone of a boss with a hardcoded item drop, such as Gish or one of the Harbingers, the second item pedestal will be from the boss item pool instead of the boss' regular drop.
Managing to defeat the boss before the twin has transformed into it will not prevent the twin from doing the transformation, and the boss fight will continue.
Obtaining Vanishing Twin in the boss fight itself will not take effect immediately. The boss room must be re-entered in order for the twin to transform.
IV - The Emperor?: The twin will clone the additional boss and spawn an item if the original one has not been visited yet. However, this will not allow for another item to be obtained from the floor's original boss, because the twin won't be active until the next floor.
If Tainted Lazarus has used Flip after the boss has been cloned, it will not drop an additional item.
A Vanishing Twin is a fetus that dies during gestation and is reabsorbed by its twin, hence the appearance of the item.
The familiar's flat appearance is due to a real life phenomenon known as fetus papyracaeus, where instead of a vanishing twin being physically reabsorbed by the other, the body stays in the womb and gets crushed and flattened by the healthy twin in full term between the fetus' expanding membranes and the uterine walls.
If Great Gideon is encountered with this item, the fight gets stuck at the second wave because the clone is counted as an alive enemy, resulting in a softlock.
Some familiars like Headless Baby will follow the other boss as if it was a familiar.
After beating the boss, familiars that were after Vanishing Twin will get bugged and stay at the entrance of every room instead of following the player.
Using IV - The Emperor while in a boss fight will immediately kill the twin boss and spawn the second item for free; the player will only have to fight one boss thereafter.
In Double Trouble boss fights, only one of the bosses will get copied, but both of them will get their health lowered down to 75%.