Renewable Power to Ammonia (RePtA) Virtual Power Plants (VPPs) are garnering global interest. However, the variability of renewable energy and the stochastic nature of forecast errors pose significant challenges in maintaining hydrogen and power balance. If we only consider the load range of 30%-100% for the Ammonia Synthesis Reactor (ASR) in normal mode, it will lead to excessive electricity trading with the power grid, thereby significantly reducing economic efficiency. Hence, we introduce two temporary operating modes for the ASR to improve the RePtA VPP's flexibility: standby and overloading. These modes are beneficial in addressing hydrogen and power imbalances, especially when hydrogen stocks are depleted or oversupplied. Furthermore, we analyze the multi-stage dispatchable regions (DRs) for the RePtA VPP and develop a DR-guided adaptive mode-switching regulation. This DR-guided strategy aids in reducing electricity costs and avoids overreaction in mode-switching. Our case studies based on an actual project in northeast China indicate that this strategy notably reduced electricity purchases and sales by 59.8% and 67.7% compared to scenarios without mode-switching. Moreover, it effectively decreased the frequency of ASR transitions to standby mode by 62.8% compared to the bang-bang control method.