It is not really Vassals job to enforce any Rules or Scenario constraints. Any such rules would have to be coded in by both the Module Creator and the Scenario Creator, adding layers of complexity that would not only increase the work load but would also detract from a couple of the most valuable features of Vassal.
1) Vassal is a great tool for Scenario Design, where the Play Testers may wish to adjust Scenario Constraints and be able to test house/scenario Rules without constraints imposed by the Programmer.
2) When teaching new players it is common, to give them a larger hand or to reduce the experienced players hand, as a means of Scenario balancing.
It is best not to think of Vassal as a Computer Game, but rather a digital representation of the physical game. The onus is on you to ensure you play by the Rules or bend them with the agreement of both players.