I plan to use this method to protect my LiPo battery pack. Does it make sense to monitor vcc to determine if there is enough power in my 7.4 V battery connected to Vin ? I hope drop in Vcc signals battery undervoltage but I have doubts.
No it won't. The Vin voltage goes to a voltage regulator that maintains a steady Vcc voltage even as Vin is dropping over time. A LiP0 two cell battery will have a voltage output from 8.4 vdc at full charge down to
6 vdc where most recommend to stop drawing power from it. You are better off using a voltage divider and measuring the battery directly via a analog input pin to decide when to alarm or shutdown.