Vacuum currently onl deals damage upon losing the debuff. This means when refreshing it, only the last cast deals damage, while the previous casts do not.

1. Cast Vacuum on an enemy
2. Before it finishes, cast a second Vacuum on the same enemy

Result: The first Vacuum deals no damage, because the second Vacuum refreshes the debuff duration, which effectively skips the first cast's damage.

Expected: Each cast should deal its damage. The easiest way to achieve this would be to make Vacuum destroy existing Vacuum debuffs on units within the area first, and then apply a new Vacuum debuff.