I don’t think the question was about competition legality. There are other uses of VEX besides VRC. For example those funny contraptions people bring to worlds (moving sculptures, roving friendly humanoid robots, self-moving carts, …) could use phone to control them.
@RoboCatz had a valid point that even for such a use case, you’d still need a driver-controls like program to translate any phone input into motor commands, but let’s assume you already have that working with the official V5 controller. Well, the V5 VexNet radios do support bluetooh communication mode (again, not legal for competitions, but you can switch the radio to bluetoot to play with that on non-VRC use cases).
Once you have a brain accepting control commands from the controller over bluetooth, you can observe the communication and emulate it from a phone app.
I haven’t tried that for VRC/V5 yet (much anyway), but apparently the V5 bluetooth protocol is similar to that of Vex IQ Smart Radio, which is reasonably documented (including SDK and demo applications) on the Smart Radio product page, see https://www.vexrobotics.com/228-3530.html#Docs_&_Downloads
If you’re proficient in writing phone apps, you should be able to pick up from that point and write a controller emulator, or even a very different app that just happen to use the same channel for encode a control paradigm very different to 2 joysticks and a bunch of buttons…