Confirmed I also experienced this today.

ISP:
AT&T

Type of Internet Connection:
ADSL

Internet Connection Speed:
0.5 Mbps

Date & Time:
3:30 PM EST

Frequency:
Always

Character Name:
Katarh Mest

Race:
Miqo'te

World:
Lamia

Class/Level:
Warrior 56

Area and Coordinates:
Western Coerthas Highlands 11,14

Party or Solo:
Party, 2 members (chocobo)

NPC Name:
N/A

Monster Name:
Ice Commander

In-Game Time:
Approximately noon Eorzea Time

Steps:
Use Vengeance
Vulnerability buff as JP
Let Vengeance wear off
Vulnerability buff as it falls off is also in JP