Dusty It said the bootloader was already locked
I'm assuming you mean un-locked, as the bootloader being locked would prevent you from flashing.
Dusty Although the bootloader button in Vanadium is not grayed out, it's now giving a "Error: Failed to execute 'requestDevice' on 'USB': No device selected.
It sounds like the connection dropped. Are you using the cable that came with your phone? Remember to not interact with the phone during the flashing, unless the installer tells you to. Also don't unplug the cable during the flashing.
Dusty I've tried to shut the phone down to stop the looping but it won't respond to holding the power button. I believe I'm stuck in this loop.
Hold the power button for at least 30 seconds until the phone turns off. Then hold power and volume down simultaneously to boot into the bootloader interface and start the flashing again.
Dusty Pretty sure at this moment, I have a $500 brick...
You don't. Please follow the steps I provided with holding down the power button until the phone turns off.