What happens if I try to draw power from the VIN port while under USB power on an Uno?
You do run the risk of damaging the 5V regulator, yes.

Power will attempt to flow from the 5V rail (USB port) through the voltage regulator (backwards) and into the shield to power it.

As your shield doesn't mind being under-powered from 5V while you're programming the Arduino you can add a diode between 5V (anode) and Vin (cathode). This will bypass the regulator when the voltage of the 5V rail is higher than the Vin rail, and your shield will be supplied with about 4.3V (slightly higher for a schottky diode).