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.

Repro:
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.