Following solutions for your reference:
1) There are two ports can provide the power for VIM: VIN and USB-C, so you can provide the power with two separate power source. Check the Extra Power Input for Vim for details.
In this case: VIM power down only when both of the two power are dropped.
2) There is a power control signal to enable/disable the system power for VIM: PWR_EN, currently, PWR_EN will be 1 when the power AO_5V is applied, so as what RDFTKV mentioned above, the device will boot automate when the power is applied:
The PWR_EN is also available on 40-Pin GPIO, it means that you can design an external power circuit/module with the dropping detection:
- USB-C as the mainly power supply, and default as ON state.
- 4-Pin VIN as the secondary power supply, and default as OFF state.
- If the manly power drops down, the PWR_EN will be 0, in this case, the external power module will enable secondary power for the 4-Pin VIN by hardware