A 52-year-old man on ICU with septic shock has persistent hypotension despite noradrenaline 0.5 mcg/kg/min. The team initiates vasopressin at 0.03 units/min as recommended by SSC 2021. Thirty minutes later, the noradrenaline dose has been reduced to 0.3 mcg/kg/min with MAP improved to 68 mmHg. This 'catecholamine-sparing' effect of vasopressin acts through which receptor?