Inferno Crater Lake, New Zealand, is a unique natural laboratory exhibiting a 40-day hydrothermal cycle of significant water level and temperature variations, hypothesized to be driven by a shallow vapor-dominated reservoir. To investigate the dynamics of this system, we conducted a six-month continuous controlled-source electromagnetic (CSEM) survey, yielding apparent resistivity and phase tensors at a one-hour temporal resolution. Both tensors show clear variations synchronized with the lake's cycle. The apparent resistivity delineates the horizontal extent of the vapor zone, while the phase tensor is sensitive to shallower (<300 m) structures. Finite element forward modeling of the phase tensor data indicates the cycle involves a growth of the vapor zone thickness by approximately 180 m. This study provides the first high-resolution continuous monitoring of a dynamic vapor-dominated system, offering unprecedented insights into the processes governing volcanic-hydrothermal features and improving our framework for their interpretation and monitoring.