Vulnerability Up is a detrimental status effect which increases the damage taken by the affected player. It is received after being hit by an avoidable boss mechanic or if one or more people do not perform certain mechanics correctly (e.g., missing a tower soak).
The strength of the effect depends on the duty, and it is usually a stacking debuff. Max stack is 16.
Players will usually only receive this or Damage Down in a given duty; rarely both.