The trinket creates no effect if Isaac has no active item.
Active items that are reusable indefinitely without a charge bar, such as How to Jump, will allow Vibrant Bulb's effects to be permanent (until the active item is swapped out).
Vibrant Bulb is slightly less than half as powerful as Dim Bulb; however, its effects are active more often.
Glowing Hour Glass, when used, will not make the Vibrant Bulb inactive and will still provide the bonus until it has one bar of charge.
Having additional charge beyond full with The Battery does not provide an additional bonus. It does give an advantage however, since using the fully charged item once still provides Vibrant Bulb's effect.
If two active items are held (Schoolbag), the one currently active must be at full charge for the trinket to take effect (the other one doesn't need to be fully charged).
If Bethany or Tainted Bethany are holding enough soul/red hearts to charge her current active item, the Vibrant Bulb will be active, even if there are no regular charges.
If the active item is fully charged only with soul/blood charges and no regular charges, Vibrant Bulb and Dim Bulb can be both active at the same time if Isaac holds both of the trinkets.
Replacing Isaac's active item with another active item that doesn't have a charge bar won't immediately activate the trinket's effects. The effect will depend on whether or not the previous item had a full charge or not. Furthermore, if the effect is active due to the previous item having a full charge, picking up a different active item that currently has no charge won't disable the effect, and vice-versa.