A vapor compression system comprising: a compressor having a suction port and a discharge port; a refrigerant flowpath from the discharge port and returning to the suction port; a first heat exchanger along the refrigerant flowpath; a second heat exchanger along the refrigerant flowpath; a vaporizer system (22; 300), wherein the vaporizer system comprises: a first vaporizer (68A; 368A) and a second vaporizer (68B; 368B) each comprising a vessel (86; 386) having an inlet, a vapor outlet, and a liquid outlet; a mixture inlet flowpath positioned to pass a refrigerant/lubricant mixture to an interior of each vessel; a gas bypass flowpath (160, 162, 160A, 160B, 150, 350) positioned to pass a refrigerant flow from the refrigerant flowpath through a heat transfer relation with the interior of each vessel and return the refrigerant flow to the refrigerant flowpath; and one or more valves (80, 140) including one or more drain valves positioned to control flow from the liquid outlet of the first vaporizer and the liquid outlet of the second vaporizer; and a controller, configured to provide: a first operational condition comprising draining lubricant from the first vaporizer while blocking draining of lubricant from the second vaporizer; and a second operational condition comprising draining lubricant from the second vaporizer while blocking draining of lubricant from the first vaporizer. a compressor having a suction port and a discharge port; a refrigerant flowpath from the discharge port and returning to the suction port; a first heat exchanger along the refrigerant flowpath; a second heat exchanger along the refrigerant flowpath; a vaporizer system (22; 300), wherein the vaporizer system comprises: a first vaporizer (68A; 368A) and a second vaporizer (68B; 368B) each comprising a vessel (86; 386) having an inlet, a vapor outlet, and a liquid outlet; a mixture inlet flowpath positioned to pass a refrigerant/lubricant mixture to an interior of each vessel; a gas bypass flowpath (160, 162, 160A, 160B, 150, 350) positioned to pass a refrigerant flow from the refrigerant flowpath through a heat transfer relation with the interior of each vessel and return the refrigerant flow to the refrigerant flowpath; and one or more valves (80, 140) including one or more drain valves positioned to control flow from the liquid outlet of the first vaporizer and the liquid outlet of the second vaporizer; and a first vaporizer (68A; 368A) and a second vaporizer (68B; 368B) each comprising a vessel (86; 386) having an inlet, a vapor outlet, and a liquid outlet; a mixture inlet flowpath positioned to pass a refrigerant/lubricant mixture to an interior of each vessel; a gas bypass flowpath (160, 162, 160A, 160B, 150, 350) positioned to pass a refrigerant flow from the refrigerant flowpath through a heat transfer relation with the interior of each vessel and return the refrigerant flow to the refrigerant flowpath; and one or more valves (80, 140) including one or more drain valves positioned to control flow from the liquid outlet of the first vaporizer and the liquid outlet of the second vaporizer; and a controller, configured to provide: a first operational condition comprising draining lubricant from the first vaporizer while blocking draining of lubricant from the second vaporizer; and a second operational condition comprising draining lubricant from the second vaporizer while blocking draining of lubricant from the first vaporizer. a first operational condition comprising draining lubricant from the first vaporizer while blocking draining of lubricant from the second vaporizer; and a second operational condition comprising draining lubricant from the second vaporizer while blocking draining of lubricant from the first vaporizer.