The vine plants tried to resist by killing more people. As long as they use the blood to heal, not grow, they won't be in violation of the first rule and will be able to continue to survive.
This move won't solve their main problem, as the second rule is still killing them. But it will give them time to come up with a solution.
The solution they had in mind was to kill the legendary nobles. But the legends didn't give them the opportunity to do so.
"It is forbidden for plants to move of their own initiative in this city."
"The blood in carnivorous plants in this city must burn and turn into flames."
The third rule sealed the fate of the vine plants. If they can't move, then they can't kill anyone else, so they won't be able to resist as they wither and die.
But the legends weren't satisfied with that. They also set their blood on fire. This way, they will die faster.
