Answered: PROS for VEX

PROS is a C based a language like ROBOTC. For those who don’t know about it, the site is here.
Am I allowed to use this to program for a competition? I couldn’t find anything in the rulebook or an official answer.

There are no rules preventing this, thus it is legal. However, be advised that software solutions not officially supported by VEX must still meet software inspection rules in order to be a legal and pass inspection. This includes, for example, not responding to driver control for the duration of the Autonomous Period, and passing the “Field Check” procedure as explained in the Inspection Guide.