Dress code

I was at a tournament this weekend where a student was wearing sandals and the student was told that at Worlds they would not be allowed to wear any opened toed shoes. I know FIRST has such a rule but was unable to find it in the Vex rules. Was this person correct? This was a person that represents Vex. Had it just been some random person I would not care but since it was someone with a position from Vex and I could not find it in the rules, I thought this was something that should be addressed so everyone would know one way or the other prior to getting to Worlds.