There are no rules explicitly prohibiting this. However, <G12> would very likely come into play, quoted below for reference:
<G12> Strategies aimed solely at the destruction, damage, tipping over, or Entanglement of Robots are not part of the ethos of the VEX Robotics Competition and are not allowed. However, VEX Robotics Competition In the Zone is an interactive game. Some incidental tipping, Entanglement, and damage may occur as a part of normal game play. If the tipping, Entanglement, or damage is ruled to be intentional or egregious, the offending Team may be disqualified from that Match. Repeated offenses could result in a Team being Disqualified from the remainder of the competition.
VEX Robotics Competition In the Zone is intended to be an offensive game. Teams that partake in solely defensive strategies will undergo extra scrutiny in regard to <G12>. In the case where referees are forced to make a judgment call on interaction between a defensive and offensive Robot, the referees will err on the side of the offensive Robot.
A Team is responsible for the actions of its Robot at all times, including the Autonomous Period. This goes for Teams that are driving recklessly and potentially causing damage, but also goes for Teams that drive around with a small wheel base. A Team should design its Robot such that it is not easily tipped over or damaged by minor contact.
A Robot which intentionally drives over an opponent, resulting in the opponent being damaged or entangled, would be considered a violation of <G12>. If this happens repeatedly, it could be considered a violation of <R3> and require re-inspection before being allowed to play further.
<R3> The following types of mechanisms and components are NOT allowed:
a. Those that could potentially damage playing field components.
b. Those that could potentially damage other competing robots.
c. Those that pose an unnecessary risk of entanglement.
In general, to protect themselves from DQs or damage (intentional or otherwise), we would recommend that teams do not drive on top of other robots.