I’m planning to use VIN to power a Nano project, are there any precautions needed when using VIN power and connecting to the USB input?
Yes. Vin will back feed thru the regulator & try to bring up your pc if your pc is off and connected via usb.
This will usually cook the regulator. If the reg fails shorted, as they often do, then full Vin is applied to the card & c'est fin!
I put a switch on the RED USB wire:
No, there will be no problem when using Vin. If in doubt, look at the schematic, there is a protection diode.
Nice, contradicting opinions 
Not on everything. I've seen both sides referenced. Having the switch helps. Test for yourself. Measure voltage on USB port w/o device. Then measure onboard voltage. If they're equal, then NO DIODE. If the onboard voltage is .6-.7V LOWER (4.3-4.4V) there there IS a diode.
Just looked up the Mega schematic. There is a 500mA FUSE, but no external diode.
Hi,
To be safe and sure use @madmark2150 solution.
This is mine;
If I need to I can jumper the two terminals together to use USB power or check 5V current.
Tom... 
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