Consider that deontological ethics abide by rules, motives and actions themselves. This is contrasted with consequentialism where the consequences dictate what is ethical and what is not. But, with deontological ethics, it is the moral inherent in the action itself that takes precedence in determining its ethical standard. In other words, is the act of legalizing marijuana ethical or unethical?
In the deontological context, we can ignore the consequences such as emphysema or second hand smoke; however, such considerations will be noted by opponents in a larger ethical contextual debate. So, there are also ethical arguments when dealing with the consequences, but this is more hotly debated than the deontological argument. But, in this context, one could easily make the case that during Prohibition, crime increased dramatically, so legalizing what is sold illegally will logically reduce crime. In a larger context of freedom itself, it seems ethical (in terms of free will) to allow people to ingest whatever they choose. The consequences are then their responsibility.
The most famous classical deontological argument is Kant's "categorical imperative." Basically, Kant states this as, "Act only according to that maxim whereby, you can, at the same time, will that it should become a universal law." If you legalize marijuana, you are saying that this should be a universal law. The value of focusing on the deontological argument is that it is the act itself (legalization), not the consequences, that determines whether the act is ethical. If a universal law is "thou shall not kill," a person doesn't kill someone because this is unethical. If a universal law is "legalize marijuana," you need to say what the motive is: get people high (and presumably happier), decrease crime, allow more freedom, etc. A strict deontologist would argue that legalizing a plant is completely ethical; or, at least, such an act can in no way be construed to be unethical.
Virtue ethics is more subjective, based on a case by case basis. The virtue of ethics does not depend on the act or the consequences; it depends on the character/virtue of the being. So, a knife's virtue is sharpness. Since humans are the ones to legalize marijuana, we have to determine what human virtue is. This is no easy task. But, keeping it simple, we might say a virtuous human is one who personally strives to be moral and knowledgeable while also striving to treat others as generously as he/she would have others treat himself (clearly borrowing from the Golden Rule here). Philosophers have differed on the idea of "the" human virtues, including justice, temperance, generosity, love, etc. This depends on the era and culture. Once you define human virtues, question whether the virtuous human would legalize marijuana. This is subjective, so it makes the argument for legalization a bit easier, but the same could be said for opponents of legalization. A just, loving, honest, generous person might say that legalization is ethical because it adds a freedom of choice and because smoking marijuana induces happiness and camaraderie. This leads to personal and social well being: more conducive to virtuous traits.
Perhaps a statistical analysis of virtue ethics may be more effective. One could measure the agreed upon virtues of those who use marijuana versus those who drink alcohol.