Using USB port while powered via VIN?

It would be really convenient for me to power a 3rd party device I am using via the 1500's USB connector. I am already, however, powering the unit via VIN (from a regulated 5V switching supply) that I've mounted on my mated MKR Relay Shield board from the small proto section. I believe the documentation stated somewhere that if powering the 1500 from VIN, then it would disable the USB port. Is there a way to bypass this, or did I misinterpret?

Otherwise, I assume I am just going to have to solder a connection from 3rd party device to the 1500's pins 13/14 UART.

The Vin power is separated from the USB power by a protection circuit, many times a diode. The one supplying the higher voltage will take over and power the board. If you are not using the A/D inputs no problem but if so you will have a different reference voltage depending on what is powering it. This only affects the power, not the D+. D-, and ground connections so the USB port should work just fine. Check the schematic on the board you are using go be sure, you do not want to be putting power back into your computer via the USB.

I was under the impression there was some sort of LDO/switcher regulating the "circuit" voltage to 3.3V nonetheless. Did not realize there may be a chance that, due to powering from VIN @ 5V, I might get 5V out of a digital pin if configured as an output... assuming I'm not misinterpreting what you're saying.

Assuming you are talking arduino UNO or = you need at least 6V for Vin. The product page of the Arduino Uno tells you, the voltage for Vin needs to be between 6-20V but the recommend 7-12V. 12V is fine if you do not add much load to the arduino voltage outputs. Many of us use something in the 7-8 volt range to keep the regulator cool. The standard UNO uses a linear regulator, not a switching unit. The digital output will give you whatever voltage is powering the microchip, assuming a small load, not what is being supplied to the board. Telling us up front what Arduino could save us a lot of time.