You can find them in the user manual, on the vehicle, or in the NIU APP after you've bound the scooter.
SN = serial number
VIN = frame number
In the user manual:
You will find a QR Code that holds the SN and VIN of your scooter. The SN is located at the top while the VIN is at the bottom.
In the vehicle:
For mopeds: There is a sticker with the SN under the seat part. The VIN is engraved on the scooter's frame and also under the seat part.
For kickscooters: You can find the SN on the side of the mainframe. It's under a barcode. The VIN starts with 199 and is located near the front wheel.
In the NIU APP:
In the tab 'Me', click on your device. You'll find SN and VIN there.