Well you are justifying banning all sorts of things because people *might* do something wrong because of it.
In all matters of whether or not something should be banned, you have to balance the probabilites of each outcome.
If you drive a car (provided that you have passed the driving test), the total benefit of choosing to drive instead of walking is believed to outweigh the negative effects, which is why it is not banned. Although there is a chance that you will hit something/someone, and that is very bad, the chances are sufficiently against it that the benefits of driving outweigh these chances.
If, however, you are under the influence of alcohol, for example, then the risk of something bad happening increases, and after a point the risk outweighs the benefits. At that point, driving is banned.
The above is an instance of banning something because people might do something wrong. If you're drunk and you drive, there's no guarantee that you'll hit somebody. However, there is sufficient chance that it is banned.
And I'm focusing on what you're saying. Basically, you're blaming the substance rather than the person. I reckon that violent people would still be violent even without any drugs.
It's not as simple as "violence is caused by violent tendencies and nothing else". Without drugs violence would still occur, but it would almost certainly be reduced.
I mean, fast food, coffee, video games and tons of other everyday things are clearly bad for you, but how would you feel if the man took them all away from you?
They may have negative effects, but the negative effects are generally believed to be outweighed by the enjoyment that people get from them. These vary from person to person, and there are some people (the biggest example is young children) who it is deemed would have the negative effects outweigh the positive ones, which is why they are controlled (with things such as age ratings).
And what's good and what's bad for you isn't really a clear-cut issue.
That's one of the major functions of a government - to decide what is good for people and what is bad for them, and act appropriately. Governments in developed countries today often won't take any action until they have performed an in-depth study of the issue to weigh up all the factors involved.
Returning to the issue of government vaccinations against drugs, let me know what precise step(s) in the following chain of thought you disagree with:
-people gain short term enjoyment from taking some drugs
-people gain long term disadvantages from taking those same drugs
-for some drugs, the disadvantages outweigh enjoyment is all or almost all cases
-when deciding whether or not to take drugs, some people misjudge the relative value of the short term enjoyment and the long term disadvantages
-therefore, those people take drugs when they should not
-therefore, those people have negative benefit from taking drugs
-the purpose (or one of the purposes) of the government is to maximise the total benefit in society in the long term
-therefore, if an action results in negative benefit, the government must attempt to reduce the effects of this negative benefit until the point where the costs of reducing it further outweigh the amount it would be reduced by.
-people taking these drugs results in negative benefit, so the government must, if the costs of doing so are not outweighed by the long term increase in benefit, do at least one of these three things:
--Increase the enjoyment of the drugs until it reaches the point where the enjoyment outweighs the long term negative effects
--Decrease the negative effects until they are outweighed by the enjoyment
--Decrease the number of people who are taking drugs
-Now, consider a vaccination against these drugs
-By vaccinating someone against these drugs, they do not gain the benefits of taking them
-with the benefits gone, they have no incentive to take drugs
-therefore, even those that are prone to misjudging the effects will not take drugs
-so the total negative benefits will decrease
-so, provided both of the following hold:
--the vaccine works
--the cost of giving people the vaccine is more cost-effective than increasing the benefits or decreasing the negative effects of the drugs-it is logical for the government to try to vaccinate as many people as possible
note that I am only considering those who do not have an addiction to drugs in the above.